Response of soil CO2 emissions and water-carbon use efficiency of winter wheat to different straw returning methods and irrigation scenarios

BACKGROUND

The shortage of water resources and the increase of greenhouse gas emissions from soil seriously restrict the sustainable development of agriculture. Under the premise of ensuring a stable yield of winter wheat through a reasonable irrigation scenario, identifying a suitable straw returning method will have a positive effect on agricultural carbon sequestration and emission reduction in North China Plain.

RESULTS

Straw burying (SR) and straw mulching (SM) were adopted based on traditional tillage under in the winter wheat growing season of 2020-2021 and 2021-2022. Three irrigation scenarios were used for each straw returning method: no irrigation (I0), irrigation 60 mm at jointing stage (I1), and irrigation of 60 mm each at the jointing and heading stages (I2). Soil moisture, soil respiration rate, cumulative soil CO2 emissions, yield, water use efficiency (WUE) and soil CO2 emission efficiency (CEE) were mainly studied. The results showed that, compared to SM, SR improved the utilization of soil water and enhanced soil carbon sequestration. SR reduced soil respiration rate and cumulative soil CO2 emissions in two winter wheat growing seasons, and increased yield by increasing spike numbers. In addition, with an increase in the amount of irrigation, soil CO2 emissions and yield increased. Under SR-I1 treatment, WUE and CEE were the highest. SR-I1 increases crop yields at the same time as reducing soil CO2 emissions.

CONCLUSION

The combination of SR and irrigation 60 mm at jointing stage is a suitable straw returning irrigation scenario, which can improve water use and reduce soil CO2 emission in NCP. © 2023 Society of Chemical Industry.

Effects of depth of straw returning on maize yield potential and greenhouse gas emissions

Appropriate straw incorporation has ample agronomic and environmental benefits, but most studies are limited to straw mulching or application on the soil surface. To determine the effect of depth of straw incorporation on the crop yield, soil organic carbon (SOC), total nitrogen (TN) and greenhouse gas emission, a total of 4 treatments were set up in this study, which comprised no straw returning (CK), straw returning at 15 cm (S15), straw returning at 25 cm (S25) and straw returning at 40 cm (S40). The results showed that straw incorporation significantly increased SOC, TN and C:N ratio. Compared with CK treatments, substantial increases in the grain yield (by 4.17~5.49% for S15 and 6.64~10.06% for S25) were observed under S15 and S25 treatments. S15 and S25 could significantly improve the carbon and nitrogen status of the 0-40 cm soil layer, thereby increased maize yield. The results showed that the maize yield was closely related to the soil carbon and nitrogen index of the 0-40 cm soil layer. In order to further evaluate the environmental benefits of straw returning, this study measured the global warming potential (GWP) and greenhouse gas emission intensity (GHGI). Compared with CK treatments, the GWP of S15, S25 and S40 treatments was increased by 9.35~20.37%, 4.27~7.67% and 0.72~6.14%, respectively, among which the S15 treatment contributed the most to the GWP of farmland. GHGI is an evaluation index of low-carbon agriculture at this stage, which takes into account both crop yield and global warming potential. In this study, GHGI showed a different trend from GWP. Compared with CK treatments, the S25 treatments had no significant difference in 2020, and decreased significantly in 2021 and 2022. This is due to the combined effect of maize yield and cumulative greenhouse gas emissions, indicating that the appropriate straw returning method can not only reduce the intensity of greenhouse gas emissions but also improve soil productivity and enhance the carbon sequestration effect of farmland soil, which is an ideal soil improvement and fertilization measure.

Effects of Straw Returning and New Fertilizer Substitution on Rice Growth, Yield, and Soil Properties in the Chaohu Lake Region of China

Recently, replacing chemical fertilizers with straw returning and new fertilizers has received considerable attention in the agricultural sector, as it is believed to increase rice yield and improve soil properties. However, less is known about rice growth and soil properties in paddy fields with the addition of different fertilizers. Thus, in this paper, we investigated the effects of different fertilizer treatments, including no fertilization (CK), optimized fertilization based on the medium yield recommended fertilizer amount (OF), 4.50 Mg ha-1 straw returning with chemical fertilizers (SF), 0.59 Mg ha-1 slow-release fertilizer with chemical fertilizers (SRF), and 0.60 Mg ha-1 water-soluble fertilizer with chemical fertilizers (WSF), on rice growth, yield, and soil properties through a field experiment. The results show that compared with the OF treatment, the new SF, SRF, and WSF treatments increased plant height, main root length, tiller number, leaf area index, chlorophyll content, and aboveground dry weight. The SF, SRF, and WSF treatments improved rice grain yield by 30.65-32.51% and 0.24-1.66% compared to the CK and OF treatments, respectively. The SRF treatment increased nitrogen (N) and phosphorus (P) uptake by 18.78% and 28.68%, the harvest indexes of N and P by 1.75% and 0.59%, and the partial productivity of N and P by 2.64% and 2.63%, respectively, compared with the OF treatment. However, fertilization did not significantly affect the average yield, harvest indexes of N and P, and partial productivity of N and P. The contents of TN, AN, SOM, TP, AP, and AK across all the treatments decreased significantly with increasing soil depth, while soil pH increased with soil depth. The SF treatment could more effectively increase soil pH and NH4+-N content compared to the SRF and WSF treatments, while the SRF treatment could greatly enhance other soil nutrients and enzyme activities compared to the SF and WSF treatments. A correlation analysis showed that rice yield was significantly positively associated with tiller number, leaf area index, chlorophyll, soil NO3--N, NH4+-N, SOM, TP, AK, and soil enzyme activity. The experimental results indicate that SRF was the best fertilization method to improve rice growth and yield and enhance soil properties, followed by the SF, WSF, and OF treatments. Hence, the results provide useful information for better fertilization management in the Chaohu Lake region of China.

Impact of Straw Incorporation on the Physicochemical Profile and Fungal Ecology of Saline-Alkaline Soil

Improving the soil structure and fertility of saline-alkali land is a major issue in establishing a sustainable agro-ecosystem. To explore the potential of different straw returning in improving saline-alkaline land, we utilized native saline-alkaline soil (SCK), wheat straw-returned saline-alkaline soil (SXM) and rapeseed straw-returned saline-alkaline soil (SYC) as our research objects. Soil physicochemical properties, fungal community structure and diversity of saline-alkaline soils were investigated in different treatments at 0-10 cm, 10-20 cm and 20-30 cm soil depths. The results showed that SXM and SYC reduced soil pH and total salinity but increased soil organic matter, alkali-hydrolyzable nitrogen, available phosphorus, total potassium, etc., and the enhancement effect of SYC was more significant. The total salinity of the 0-10 cm SCK soil layer was much higher than that of the 10-30 cm soil layers. Fungal diversity and abundance were similar in different soil layers in the same treatment. SXM and SYC soil had higher fungal diversity and abundance than SCK. At the genus level, Plectosphaerella, Mortierella and Ascomycota were the dominant groups of fungal communities in SXM and SYC. The fungal diversity and abundance in SXM and SYC soils were higher than in SCK soils. Correlation network analysis of fungal communities with environmental factors showed that organic matter, alkali-hydrolyzable nitrogen and available phosphorus were the main environmental factors for the structural composition of fungal communities of Mortierella, Typhula, Wickerhamomyces, Trichosporon and Candida. In summary, straw returning to the field played an effective role in improving saline-alkaline land, improving soil fertility, affecting the structure and diversity of the fungal community and changing the interactions between microorganisms.

[Effects of Straw Returning with Lime on SOC and Carbon Pool Management in Acidic Paddy Soil]

Soil acidification and low SOC are the main limiting factors in acidic paddy soils. Straw returning with lime is an effective measure to alleviate soil acidification and improve soil fertility; however, its interaction effects on SOC and carbon pool management are still unclear. To investigate the impact of straw returning with lime on the organic carbon pool of acidic paddy soil, field experiments were conducted on acidic paddy soil in Baiyun District and Huiyang District of Guangdong Province. The changes in soil total organic carbon (TOC), water-soluble organic carbon (DOC), active organic carbon (LOC), particulate organic carbon (POC), microbial biomass carbon (MBC), carbon pool index (CPI), stable organic carbon (IOC), carbon pool activity (L), carbon pool activity index (CPAI), and carbon pool management index (CPMI) were analyzed under three treatments (CK, conventional fertilization; RS, straw returning+conventional fertilization; RS+L straw returning with lime+conventional fertilization). The results demonstrated that compared with that in CK, the TOC, LOC, POC, and MBC in the RS+L treatment were significantly increased by 10.24%-17.79%, 34.49%-44.37%, 19.27%-23.59%, and 33.36%-43.26%, respectively (P<0.05). Compared with that in CK, the RS+L treatment significantly increased the DOC content during the early growth stage (15-45 days after transplanting) of rice (P<0.05) but had no significant influence on the DOC content during the late growth stage of rice. Compared with that in RS, the TOC, LOC, POC, and MBC in the RS+L treatment were increased by 2.15%-6.95%, 1.17%-17.90%, 4.27%-8.65%, and 12.99%-14.53%, respectively. Compared with that in CK, the RS+L treatment significantly increased IOC and CPI by 8.32%-15.57% and 14.00%-20.00%, respectively (P<0.05). Compared with that in the CK treatment, the RS treatment significantly increased CPI by 14.00%-18.00% (P<0.05). No significant differences in L, CPAI, or CPMI were detected among the different treatments. The soil pH in the RS+L treatment was significantly higher than that in the CK treatment (P<0.05). No significant differences in rice yield were detected among the different treatments. Principal component analysis demonstrated that rice yield was primarily correlated with DOC, LOC, CPAI, and CPMI but its contribution to SOC and carbon pool management index was low. Principal component analysis also indicated that straw returning with lime could improve soil pH and nutrient contents of acidic paddy soil, driving the formation and accumulation of organic carbon fraction such as MBC and POC, thus boosting the increase in SOC. In conclusion, straw returning with lime is beneficial to the accumulation of MBC, POC, LOC, and IOC in acidic paddy soil to improve the content and stability of soil total organic carbon, which is an effective way to improve the carbon sequestration of acidic paddy soil.

[Effects of Aeration on Surface Water Nutrient Dynamics and Greenhouse Gas Emission in Different Straw Returning Paddy Fields]

Straw returning is of great significance for improving soil structure, soil fertility, crop yield, and quality. However, straw returning causes environmental problems such as increased methane emission and non-point source pollutant emission risk. How to reduce the negative effects of straw returning is an urgent problem to be solved.In this study, the effects of aerobic treatment on carbon and nitrogen concentration in surface water and greenhouse gas emissions in paddy fields with different treatments of straw returning were systematically compared.The results showed that different treatments of straw returning significantly increased chemical oxygen demand (COD) in the surface water of the paddy field and significantly promoted the methane emission of the rice field and the global warming potential (GWP), although it slightly reduced N₂O emission. The increasing trends showed that wheat straw returning>rape straw>broad bean straw returning.Straw returning increased rice yield when compared with the control without straw returning, but the difference was not significant. Aerobic treatment reduced the COD in surface water by 15%-32%, the methane emission of the paddy field by 10.4%-24.8%, and the GWP of paddy field by 9.7%-24.4% under different straw returning treatments, without affecting the rice yield. The mitigation effect of aerobic treatment with wheat straw returning was the best. The results indicated the potential of oxygenation measures in greenhouse gas emission mitigation and COD emission risk reduction in straw returning paddy fields, especially in wheat straw returning paddy fields.

Analysis of Three-Way Game of Straw Return System under the Green Transformation of Agriculture

Open burning of straw is the most significant problem of environmental pollution in rural areas. Returning straw to the fields is beneficial to rural environmental management and rural development. Comprehensive utilization of straw in the field not only reduces environmental pollution, but also benefits food production and farmers' income. Because planting farmers, enterprises, and local governments have different interests, it is difficult for the straw return system to operate soundly. In this study, a three-party evolutionary game model of farmers, enterprises, and local governments was constructed to analyze the evolutionary stability of the strategic choices of the three subjects, explore the influence of each element on the strategic choices of the three parties, and use Matlab2022b simulation to further analyze the dynamic evolution of the game behavior of the system subjects under the given benefits and the given subjects, respectively. The study findings showed that the higher the preferences given by the local government, the higher the probability of farmers and enterprises participating in the straw return system. Only with the participation of local governments can the straw return system be operated robustly. Our study findings also revealed that the interests of farmers must be fully protected in order to mobilize the main body and stimulate market dynamics. The overall findings of this study provide useful insights for promoting government agencies to govern the local environment, increase local revenues, and build integrated waste utilization systems.

Dynamic variation of bacterial community assemblage and functional profiles during rice straw degradation

Bacteria is one of the most important drivers of straw degradation. However, the changes in bacterial community assemblage and straw-decomposing profiles during straw decomposition are not well understood. Based on cultivation-dependent and independent technologies, this study revealed that the "common species" greatly contributed to the dynamic variation of bacterial community during straw decomposition. Twenty-three functional strains involved in straw decomposition were isolated, but only seven were detected in the high-throughput sequencing data. The straw decomposers, including the isolated strains and the agents determined by functional prediction, constituted only 0.024% (on average) of the total bacterial community. The ecological network showed that most of the identified decomposers were self-existent without associations with other species. These results showed that during straw composition, community assembly might be greatly determined by the majority, but straw decomposition functions might be largely determined by the minority and emphasized the importance of the rare species in community-specific functions.

Effects of straw returning combined with earthworm addition on nitrification and ammonia oxidizers in paddy soil

Introduction

Soil ammonia oxidation, which acts as the first and rate-limiting step of nitrification, is driven by ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and complete ammonia oxidizer (comammox, amoA gene of clade-A and clade-B). Straw returning, widely used ecological technology in China, is an effective measure for promoting straw decomposition and soil nutrient cycling when combined with earthworm addition. However, the effects of straw returning combined with earthworm addition on soil ammonia oxidizers remain poorly understood.

Methods

A 2-year plot experiment was conducted with 5 treatments: no fertilizer (CK); regular fertilization (RT); straw returning (SR); earthworm addition (W); straw returning + earthworm addition (SRW). The AOA, AOB, comammox clade-A and clade-B community microbial diversities and structures were investigated by high-throughput sequencing.

Results

The results showed that (1) compared to RT treatment, W, SR, and SRW treatments all significantly increased the richness of AOA and comammox clade-A and clade-B (p < 0.05), and the richness of AOB was only significantly promoted by SRW treatment (p < 0.05). However, only SRW had a higher comammox clade-B diversity index than RT. (2) The ammonia oxidizer community structures were altered by both straw returning and earthworm addition. Soil NH4 +-N was the critical environmental driver for altering the ammonia oxidizer community structure. (3) Compared with RT treatment, the soil potential nitrification rate (PNR) of W and SRW treatments increased by 1.19 and 1.20 times, respectively. The PNR was significantly positively correlated with AOB abundance (path coefficient = 0.712, p < 0.05) and negatively correlated with clade-B abundance (path coefficient = -0.106, p < 0.05).

Discussion

This study provides scientific support for the application of straw returning combined with earthworm addition to improve soil nitrification with respect to soil ammonia-oxidizing microorganisms.

Characterizing corn-straw-degrading actinomycetes and evaluating application efficiency in straw-returning experiments

Corn straw is an abundant lignocellulose resource and by-product of agricultural production. With the continuous increase in agricultural development, the output of corn straw is also increasing significantly. However, the inappropriate disposal of straw results in wasting of resources, and also causes a serious ecological crisis. Screening microorganisms with the capacity to degrade straw and understanding their mechanism of action is an efficient approach to solve such problems. For this purpose, our research group isolated three actinomycete strains with efficient lignocellulose degradation ability from soil in the cold region of China: Streptomyces sp. G1^T, Streptomyces sp. G2^T and Streptomyces sp. G3^T. Their microbial properties and taxonomic status were assessed to improve our understanding of these strains. The three strains showed typical characteristics of the genus Streptomyces, and likely represent three different species. Genome functional annotation indicated that most of their genes were related to functions like carbohydrate transport and metabolism. In addition, a similar phenomenon also appeared in the COG and CAZyme analyses, with a large number of genes encoding carbohydrate-related hydrolases, such as cellulase, glycosidase and endoglucanase, which could effectively destroy the structure of lignocellulose in corn straw. This unambiguously demonstrated the potential of the three microorganisms to hydrolyze macromolecular polysaccharides at the molecular level. In addition, in the straw-returning test, the decomposing consortium composed of the three Streptomyces isolates (G123) effectively destroyed the recalcitrant bonds between the various components of straw, and significantly reduced the content of active components in corn straw. Furthermore, microbial diversity analysis indicated that the relative abundance of Proteobacteria, reportedly associated with soil antibiotic resistance and antibiotic degradation, was significantly improved with straw returning at both tested time points. The microbial diversity of each treatment was also dramatically changed by supplementing with G123. Taken together, G123 has important biological potential and should be further studied, which will provide new insights and strategies for appropriate treatment of corn straw.

[Effects of Continuous Straw Returning with Chemical Fertilizer on the Carbon Pool and Crop Yield of Rice-Rape Rotation Soils]

According to the positioning experiment of straw returning in the continuous field 7a, the effects of straw returning combined with chemical fertilizer on soil total organic carbon (TOC), dissolved organic carbon (DOC), particulate organic carbon (POC), labile organic carbon (LOC), carbon pool management index (CPMI), and crop yield in farmland soil profiles (0-20, 20-50, and 50-80 cm) in the Chaohu Lake area were studied. There were four treatments:no straw returning+no fertilization (CK), conventional fertilization (F), straw returning+conventional fertilization (SF1), and straw returning+80% conventional fertilization (SF2). The changes in soil total organic carbon and component content, CPMI, and rape rice yield in different soil layers were analyzed. Taking CK as a reference, conventional fertilization and straw returning combined with chemical fertilizer increased the content of total organic carbon and components in the soil vertical profile, and the content of total organic carbon and components in different soil layers decreased gradually with the increase in soil depth. In the 0-20 cm soil layer, compared with that in the F treatment, the SF1 and SF2 treatments significantly increased the contents of TOC, DOC, POC, and LOC by 14.23%-28.97%, 7.86%-27.01%, 16.46%-24.24%, and 5.89%-6.64%, respectively (P<0.05). In the 20-50 cm soil layer, the contents of TOC and LOC in SF1 were significantly increased by 9.43% and 8.34%, respectively, compared with those in the F treatment (P<0.05), and the contents of DOC and POC in SF2 were significantly increased by 17.51% and 65.83% compared with those in the F treatment (P<0.05). In the 50-80 cm soil layer, there was no significant difference in the contents of total organic carbon and components among the treatments. The effect of straw returning and chemical fertilizer on the soil carbon pool management index was significant. SF1 significantly improved the CPMI of the 0-50 cm soil layer compared with that in the F treatment, whereas the CPMI of the F treatment was the largest in the 50-80 cm soil layer; however, there was no significant difference among all treatments. Straw returning combined with chemical fertilizer had a significant effect on crop yield, and the yield of the SF1 treatment was the highest; compared with that of the F treatment, the rice, rape, and annual yields were significantly increased by 6.19%, 7.67%, and 6.54%, respectively (P<0.05). In general, straw returning combined with chemical fertilizer was of great significance to improve the soil carbon pool, soil fertility, and crop yield in the Chaohu Lake area.

[Effects of Long-term Straw Returning on Fungal Community, Enzyme Activity and Wheat Yield in Fluvo-aquic Soil]

To illustrate the effects of long-term straw returning on the fungal community, soil enzyme activity, and crop yield in a fluvo-aquic soil area typical of the Huang-Huai-Hai Plain, a 10-year field experiment (established in 2010) located in Dezhou City, Shandong province, was performed, including three fertilization regimes (NF, no fertilization control; NPK, fertilization with chemical N, P, and K fertilizers; NPKS, straw returning combined with chemical N, P, and K fertilizers). This study aimed to explore the regulation mechanisms of fungal communities on soil fertility, enzyme activities, and crop yield by employing co-occurrence network and structural equation model analyses. Our results showed that long-term straw returning significantly improved soil nutrients, enzyme activity, and wheat yield. Compared with the NPK and NF treatments, soil organic matter (SOM) increased by 9.20% and 34.75%, alkali-hydrolyzed nitrogen (AN) increased by 12.03% and 39.17%, dehydrogenase (DHA) increased by 37.21% and 50.91%, β-glucosidase (β-GC) increased by 17.29% and 73.48%, and wheat production increased by 16.22% and 125.53%, respectively. Different long-term fertilization regimes did not significantly change soil fungal α-diversity but resulted in significant differences in β-diversity. Available phosphorus (AP), SOM, and AN were the main driving factors of fungal community differentiation based on redundancy analysis and hierarchical partitioning analysis. Different abundance analyses revealed significantly different fungal community compositions among fertilization regimes. The long-term NF treatment resulted in a significant enrichment of phosphate/potassium-solubilizing species (i.e., Mortierella, Aspergillus, Ceriporia, and Acremonium) and symbiotic species (i.e., Leohumicola and Hyalodendriella). The relative abundance of pathogenic fungi, namely Sarocladium, Fusarium, and Fusicolla, increased significantly in the NPK treatment. Long-term straw returning in the NPKS treatment significantly stimulated the growth of plant growth-promoting species (i.e., Pseudogymnoascus and Schizothecium) and straw-degrading species (i.e., Trichocladium and Lobulomyces). Co-occurrence network analysis showed that the fungal network was composed of four main modules; the cumulative relative abundance of module 2 was significantly increased under the NPKS treatment and showed a positive linear correlation with DHA and β-GC. The structural equation model further indicated that the wheat yield was mainly regulated by SOM, whereas species of module 2 could indirectly affect SOM and wheat yield by positively regulating DHA and β-GC. Taken together, long-term straw returning to the fluvo-aquic soil area of the Huang-Huai-Hai Plain could regulate fungal interspecific interactions, stimulate the growth of specific species groups, inhibit the activity of pathogens, increase the activity of soil enzymes, promote the accumulation of SOM, and achieve high crop yield.

[Effects of Early Rice Straw Returning with Reducing Potassium Fertilizer on Late Rice Yield and Soil Fertility]

Rice straw is an important organic fertilizer in the region for double-cropping rice in South China. To reveal the effects of early rice returning with reducing potassium fertilizer on the yield of late rice and soil fertility, field experiments were carried out in Baiyun and Huiyang district in Guangdong province. The biomass, K content, and yield of late rice and the soil fertility properties, such as soil available potassium, soil organic carbon, bacterial diversity, and bacterial community structure were analyzed under three treatments (CK, conventional fertilization; RS, straw returning with conventional fertilization; RS-K, straw returning with reducing 20% potassium fertilizer). The results showed no significant differences in the biomass and yield of late rice between the RS-K treatment and CK treatment. Compared with that in CK, the RS treatment significantly increased the K contents of rice by 3.97% (Baiyun) and 6.91% (Huiyang). The K contents of late rice under the RS-K treatment were significantly lower than that under the CK treatment during the early growth period in rice, but there was no significant difference between them during the late growth period. Compared with that in CK, the soil available K in the RS treatment increased by 13.90% (Baiyun) and 21.67% (Huiyang) (P<0.05), and the soil available K in the RS-K treatment also increased by 3.56% (Baiyun) and 4.23% (Huiyang). Compared with that in the CK treatment, the soil dissolved organic carbon increased significantly in the RS and RS-K treatments (P<0.05). Compared with that in CK, the straw returning treatments (RS and RS-K) significantly improved the Chao1 and Shannon indexes of soil bacteria (P<0.05). Straw returning treatments (RS and RS-K) increased the relative abundance of Proteobacteria, Actinobacteria, and Nitrospirae compared with that in CK, whereas they decreased the relative abundance of Acidobacteria, Bacteroidetes, and Firmicutes. Redundancy analysis showed that the soil bacterial community was mainly influenced by soil organic carbon, dissolved organic carbon, microbial biomass carbon, available P, and available K. In summary, early rice returning could increase soil available K and K content in late rice. Early rice straw returning with reducing potassium fertilizer had no negative impacts on the growth and yield of late rice and could also improve soil organic carbon and the diversity of soil bacteria. Therefore, early rice straw returning with reducing potassium fertilizer can guarantee the grain yield of late rice and improve soil fertility.

[Interannual variation of soil organic nitrogen fractions and its response to straw returning.]

Elucidating the interannual variation of soil organic nitrogen fractions and its response to straw returning is of great significance for rational regulation of soil organic nitrogen pool and sustainable soil utilization. We conducted a field microcosm experiment with typic hapludoll soil at the National Field Observation and Research Station of Shenyang Agroecosystems. Three treatments were set, including nitrogen fertilizer addition (200 kg N·hm^-2, the same in other treatments), nitrogen fertilizer addition with 50% straw return, and nitrogen fertilizer addition with 100% straw return. We classified soil organic nitrogen fractions in the 1st, 3rd, 6th, and 9th years of the experiment by using the Bremner acid hydrolysis method. The results showed that the content of amino acid nitrogen increased with the tillage years, with an increase rate of 39.8% compared with 1st year. The content of hydrolyzable unknown nitrogen increased by 10.8% compared with 1st year, which reached the highest in the 3rd year. The content of total soil nitrogen and other organic nitrogen fractions showed limited variation with tillage years. The proportion of hydrolyzable total nitrogen that is relatively easy to mineralize in the total soil nitrogen gradually increased with the tillage years, and that of relatively stable acid insoluble nitrogen to total soil nitrogen gradually decreased, indicating that soil nitrogen availability increased with the tillage years, which would facilitate the soil nitrogen supply capacity. Compared with the treatment without straw returning, adding straw improved soil total nitrogen and each hydrolyzable nitrogen contents, with such positive effect be stronger under the treatment with heavier straw returning. The effect of straw returning on hydrolyzable nitrogen fractions mainly occurred in the 6th and 9th years. The components of soil total nitrogen that have been increased were mainly the amino acid nitrogen and hydrolyzed unknown nitrogen, resulting in increased proportion of hydrolyzable nitrogen. Straw returning could increase soil nitrogen pool and improve soil nitrogen conservation and supply capacity.

Method of straw ditch-buried returning, development of supporting machine and analysis of influencing factors

This paper aims to solve the problems of the low quality and shallow depth of the traditional straw return method. According to the requirements of the new furrow burial and return agronomic model, a corn straw ditch-buried returning machine was designed that could simultaneously complete the processes of picking, conveying, ditching, soil-covering and pressing. Key components were theoretically analyzed and designed, such as the pickup device, ditching device and straw-guiding soil-covering and pressing device. Based on a field experiment, the main factors influencing the effects of straw picking, soil ditching and straw return were studied. Both forward speed and pickup device speed significantly affected the straw picking rate. The ditching area, ditching width consistency factor and ditching depth stability factor gradually decreased with increasing forward speed and gradually increased with increasing trenching device speed. There was a significant interaction among the forward speed, pickup device speed and ditching device speed. At a forward speed of 1.68 m/s, the picking device speed was 330 r/min, the ditching device speed was 290 r/min, and the highest straw return rate was 93.65%.

[Effects of Straw Returning and Fertilizer Application on Soil Nutrients and Winter Wheat Yield]

In order to explore the effects of straw returning combined with fertilizer on soil nutrients and winter wheat yield in the Guanzhong area, an experimental split plot design was utilized. The main plot consisted of no straw returning (S₀) and straw returning (S). The sub-regions consisted of no fertilizer (WF), nitrogen fertilizer (NF), and nitrogen and phosphate fertilizer (NPF). Ecological stoichiometry was used to study the relationship between soil carbon, nitrogen, phosphorus content, and yield under straw returning combined with nitrogen and phosphorus fertilizer conditions. The results showed that straw and fertilization interactions had significant effects on soil organic carbon, total nitrogen, and total phosphorus contents in the surface layer (0-20 cm) (P<0.05). Compared with that in the S₀WF treatment, the SNPF treatment significantly increased soil organic carbon and total nitrogen contents in the surface layer (0-20 cm) (P<0.05). The interaction between straw and year had significant effects on soil total nitrogen content in the surface layer (0-20 cm) (P<0.05). With the increase in straw returning time, the total nitrogen content of soil 0-20 cm under the SWF treatment was significantly higher than that under the S₀WF treatment (P<0.05). Straw and fertilization and their interaction had no significant effects on organic carbon and total nitrogen contents in the 20-40 cm soil layer (P>0.05). Straw and straw interaction with fertilization significantly affected total P content in 20-40 cm soil (P<0.05). Compared with that in the SWF treatment, the SNPF treatment significantly increased the total phosphorus content in the 20-40 cm soil layer (P<0.05). Straw returning combined with chemical fertilizer also had a significant effect on soil stoichiometry. Compared with that in the S₀WF treatment, the S₀NPF treatment decreased soil C:N in the surface layer (0-20 cm) and increased soil C:P and N:P in the surface layer (0-20 cm). Compared with that in the SWF treatment, the SNF treatment reduced soil C:N in the surface layer (0-20 cm). Straw returning combined with chemical fertilizer also had a significant effect on winter wheat yield. In 2020 and 2021, the SNPF treatment increased production by 24.23% and 28.9%, respectively, compared with that of the S₀WF treatment. Correlation analysis showed that yield was significantly positively correlated with C:N (P<0.05) and C:P (P<0.01). At the same time, total nitrogen and N:P were positively correlated with treatment years (P<0.001). In conclusion, straw returning and that combined with nitrogen and phosphate fertilizer (SNPF) can improve soil nutrient characteristics, change soil stoichiometric characteristics, and increase yield in the Guanzhong area. Therefore, the results of this study indicate that straw returning combined with nitrogen and phosphate fertilizer (SNPF) is an effective way to optimize regional farmland nutrient management and improve grain production capacity.

[Effects of Biochar and Straw Returning on Soil Fungal Community Structure Diversity in Cotton Field with Long-term Brackish Water Irrigation]

Brackish water irrigation increases soil salinity and changes the soil environment, which affects the structure and diversity of soil fungi. In this study, the effects of biochar and straw (3.7 t·hm^-2 and 6 t·hm^-2, respectively) on soil physical and chemical properties and fungal community structure diversity were investigated on the basis of long-term brackish water irrigation. The results showed that compared to the absence of biochar and straw application (control), biochar application significantly increased pH and the contents of total carbon, available potassium, and available phosphorus in soil but significantly decreased the soil conductivity by 20.71%. Straw treatment significantly increased the content of available potassium and phosphorus but significantly decreased the soil bulk density and conductivity by 4.17% and 64.50%, respectively. The biochar and straw treatment showed an increasing trend in the Chao1 index and ACE index of the fungal community but a decreasing trend in the Shannon index and Simpson index. The dominant fungal phyla in the soil were Ascomycota, Mortierellomycota, Basidiomycota, Chytridiomycota, and Glomeromycota. The dominant fungal genera were Chaetomium, Gibberella, Fusarium, Idriella, and Mortierella. Biochar and straw were applied to increase the relative abundance of Ascomycota, Mortierellomycota, Basidiomycota, Glomeromycota, and Chaetomium. However, the relative abundance of Chytridomycota, Gibberella, and Idriella decreased. LEfSe analysis showed that biochar application and straw returning decreased the number of potential biomarkers in fungal communities. RDA results showed that soil fungal community structure was significantly correlated with EC_1:5 and TN. Brackish irrigation had adverse effects on soil, in which EC_1:5and TN were the main factors driving the change in soil fungal community structure. The soil fungal community adapted to a salt-stress environment through the improvement of soil by biochar and straw.

[Effect of Long-term Straw Returning on the Mineralization and Priming Effect of Rice Root-carbon]

Long-term straw returning to the field changes the environmental conditions of rice paddy soil, which affects the mineralization and priming effect of residual rice roots in the soil, but the direction and intensity of its influence is not clear. Therefore, based on a long-term fertilization field experiment, ¹³C-CO₂ isotopic labeling technology and laboratorial incubation were used to analyze the characteristics of mineralization of rice roots and native soil organic carbon, the intensity and direction of the priming effect, and the source partitioning of CO₂ emissions in three treatments, consisting of no fertilization (CK), chemical fertilizer (CF), and straw returning with chemical fertilizer (CFS). The results showed that after 120 days of flooding incubation, the root residue (R) increased the cumulative CO₂ emissions by 617.41-726.27 mg·kg^-1. The cumulative CO₂ emissions from roots and root mineralized proportions in the CFS+R and CF+R treatments were 470.82 and 444.04 mg·kg^-1, respectively, and 18.8% and 17.8%, respectively. These were significantly higher than those in the CK+R treatment (384.19 mg·kg^-1, 15.4%). There was no significant difference in the cumulative CO₂ emissions from native soil organic carbon among the three treatments. However, the mineralized proportion of native soil organic carbon in the CFS+R treatment (4.2%) was significantly lower than that in the CF+R and CK+R treatments (5.4% and 5.8%). The priming effect in the CFS+R treatment was 29.6%, which was significantly lower than that in the CK+R treatment (42.5%) and higher than that in the CF+R treatment (14.4%). A total of 23.47% to 27.59% of the cumulative CO₂ emission of the flooded paddy soil was from the roots, and the remainder was from the soil. In addition, the proportion of CO₂ emission caused by the priming effect was smaller in the CFS+R treatment than that in the CK+R treatment and larger than that in the CF+R treatment. In summary, the long-term straw returning in the flooded paddy soil will increase the mineralization potential of rice roots, but it is more conducive to the stability of the native soil organic carbon.

[Effects of Cotton Stalk Returning on Soil Enzyme Activity and Bacterial Community Structure Diversity in Cotton Field with Long-term Saline Water Irrigation]

Long-term saline water irrigation will increase soil salinity, adversely affect soil physical and chemical properties, and change the diversity of soil bacteria. Straw returning can improve the soil microenvironment and subsequently affect soil enzyme activity and bacterial community structure diversity. This experiment used two types of irrigation water salinity:fresh water (FW, 0.35 dS·m^-1) and saline water (SW, 8.04 dS·m^-1). Under each irrigation water salinity, the amount of cotton straw applied was 0 and 6000 kg·hm^-2 (represented by FWST and SWST, respectively). The results showed that:compared with those under fresh water irrigation, saline water irrigation significantly increased the soil salt, bulk density, total carbon, and available phosphorus but significantly decreased available potassium content. Under saline water irrigation, straw returning significantly increased the soil total carbon, total nitrogen, available potassium, and available phosphorus but reduced soil bulk density. Compared with those under fresh water irrigation, soil sucrase, alkaline phosphatase, and catalase activities under saline water irrigation decreased by 57.24%, 35.15%, and 3.91%, respectively, whereas urease activity increased by 26.73%. However, straw returning significantly increased sucrase, alkaline phosphatase, and catalase activities but decreased urease activity. Saline water irrigation decreased the relative abundance of Acidobacteriota, Actinobacteriota, Bacteroidota, Verrucomicrobiota, and Firmicutes and increased the abundance of Gemmatimonadota and Myxococcota. Under saline water irrigation, straw returning significantly increased the relative abundance of Actinobacteriota, Bacteroidetes, Firmicutes, Crenarchaeota, Sphingomonas, Dongia, and Steroidobacter. NMDS results also showed that saline water irrigation and straw returning changed the bacterial community structure. In conclusion, straw returning can improve soil nutrient content, reduce soil bulk density and salinity, and then change soil enzyme activity and bacterial community structure diversity. The change in soil bacterial community composition was mainly affected by soil salinity and bulk density. Therefore, straw returning can improve soil fertility and help maintain the health of soil ecosystem. This study revealed a relationship between soil enzyme activities and bacterial communities, which provides a theoretical basis and mechanism for applying cotton stalk to regulate the soil enzyme and micro-ecological environment.

[Effects of long-term straw returning on distribution of aggregates and nitrogen, phosphorus, and potassium in paddy]

In order to understand the composition and stability of soil aggregate in paddy filed, as well as the changes of soil aggregate-associated nitrogen (N), phosphorus (P) and potassium (K) after straw addition combined with chemical fertilization, soil samples were collected from a 34-year positioning experiment with three treatments, including no chemical fertilizer (CK), chemical fertilizer only (NPK), and straw addition plus chemical fertilizer (NPKS). The composition of water-stable aggregates at the soil layers of 0-20 cm and 20-40 cm were analyzed with the wet sieving method, as well as the distribution characteristics, contribution rate and activation rate of soil aggregate-associated N, P, and K. Results showed that the fractions of >2 mm and 0.25-1 mm aggregates dominated the soil water-stable aggregates in paddy field, while the contribution of <0.053 mm aggregates was lowest. Compared with CK, NPKS treatment increased the contents of >2 mm and 1-2 mm aggregates at the layers of 0-20 and 20-40 cm, and reduced the contents of 0.053-0.25 mm and <0.053 mm. Similar result in NPK treatment was observed at the layer of 0-20 cm. Compared with tat under the NPK treatment, mean weight diameter (MWD) and geometric mean diameter (GMD) increased by 3.9%-15.5% and 6.3%-41.7% in NPKS treatment, respectively. However, the unstable aggregate index (E_LT) reduced by 5.7%-28.7% in the NPKS treatment. NPKS significantly increased the contents of total N (TN), available P (AP), and available K (AK) in soil aggregates, especially in the >0.25 mm aggregates. There were no significant diffe-rences about alkali-hydrolysable N (AN) and total K (TK) between NPK and NPKS treatments. The nutrient contribution of soil aggregates in paddy field was affected by aggregate composition. NPKS significantly increased the contribution of AN, AP, and AK within >1 mm aggregates. In all, straw addition combined with chemical fertilizer could increase the stability of soil aggregates at the layers of 0-20 cm and 20-40 cm, and increase the contents of soil aggregate-associated N, P and K, especially for the >1 mm aggregates. Our results provided insights into ensuring soil quality and sustainable development of resources in paddy field by adjusting the ratio of soil C to N.

[Effects of Straw Returning with Chemical Fertilizer on Soil Enzyme Activities and Microbial Community Structure in Rice-Rape Rotation]

Straw returning is an effective technique for improving soil fertility and maintaining crop productivity in agro-ecosystems. The effects of straw returning, when combined with chemical fertilizer, on soil nutrients, enzyme activity, and microbial community were explored in rice-rape rotation farmland in the Chaohu Area. We carried out a 4-year field experiment (2016-2020) and set up four treatments (no straw+no fertilization, CK; conventional fertilization, F; straw returning+conventional fertilization, SF; and straw returning+conventional fertilization minus 20%, SDF) to explore the key environmental factors affecting soil enzyme activity and microbial and fungal communities. The results showed that straw returning combined with chemical fertilizer could improve soil nutrient content, with the SF treatment resulting in the highest soil nutrient content. Compared with F, the SF treatment significantly increased the organic matter (OM) and total phosphorus (TP) content of the soil, by 7.94% and 24.07%, respectively, in rice seasons (P<0.05), while the alkaline nitrogen (AN) content was significantly increased by 13.62% in rape seasons (P<0.05). Compared with F, the SF treatment also significantly increased soil phosphatase and urease, by 28.54% and 24.13% in rice seasons and 38.97% and 30.70% in rape seasons, respectively (P<0.05). Compared with F, SDF treatments significantly increased urease activity by 20.31% in rice seasons and 24.33% in rape seasons (P<0.05). The results indicated that straw returning increased both the Chao1 and Shannon indices of soil bacteria in rice seasons, whereas decreased these indices in rape seasons. However, the Chao1 and Shannon index of the fungal community increased after straw returning. In terms of microbial community structure, the relative abundance of Proteobacteria in SF and SDF treatments increased by 8.22% and 7.88% in rice seasons and 18.53% and 5.68% in rape seasons, respectively, compared with the F treatment. Compared with F, the relative abundance of Chloroflexi in SF and SDF treatments increased by 12.00% and 11.25% in rice seasons and 15.02% and 8.43% in rape seasons, respectively. Compared with F, the relative abundance of Basidiomycota in SF and SDF treatments in rice seasons increased by 70% and 43.42% (P<0.05), respectively, while ascomycetes in rape seasons increased by 69.79% and 43.72% (P<0.05), respectively. In conclusion, straw returning combined with chemical fertilizer can improve soil nutrient content. Soil urease and phosphatase were more sensitive to straw returning. The compositional changes in the bacterial community of the soil were mainly affected by soil TP and available phosphorus (AP), whereas OM, AN, and pH were the main environmental factors causing changes in the fungal community composition. Consequently, straw returning can improve soil fertility and maintain ecosystem health.

[Effects of straw returning and fertilization on soil fertility and yield and quality of edible sweetpotato]

Two-year field experiments were conducted in 2017-2018 to examine the effects of wheat straw returning and fertilization on soil fertility and enzyme activities, as well as the yield and qua-lity of edible sweetpoato. There were five treatments, including conventional fertilization+zero straw (CK), conventional fertilization+50% straw returning (50%S), zero fertilization+100% straw returning (100%S-F), conventional fertilization+100% straw returning (100%S), conventional fertilization+100% straw retuning+150 kg N·hm^-2 (100%S+N). The treatments of straw returning and fertilization significantly increased the contents of available phosphorus (P), hydroly-zable nitrogen (N), total N, and organic matter in soils, and increased the activities of soil catalase, alkaline phosphorylase, urease, and invertase. The storage root yield, single root fresh weight and commodity potato rate were significantly increased under the treatments of straw returning and fertilization. The storage root yield was the lowest under the treatment of 50%S. After two years of straw returning, storage root yield and commodity potato rate were the highest under the treatment of 100%S. In general, the contents of starch and protein in sweetpotato were increased after two years of straw returning and fertilization, but the contents of reducing sugar and soluble sugar were decreased under 100%S and 100%S+N treatments. Our results suggested that straw returning in full quantity was better than straw returning in half quantity. The storage root yield and commodity potato rate was the highest under the combination of full quantity straw returning and conventional fertilization, with the taste of sweetpotato being changed. Thus, the amount of nitrogen fertilizer should be appropriately reduced in actual practice.

[Dynamic Effects of Direct Returning of Straw and Corresponding Biochar on Acidity, Nutrients, and Exchangeable Properties of Red Soil]

To compare the dynamic effects of straw and corresponding biochar on soil acidity, nutrients, and exchangeable capacity in red soil, a pot experiment was performed. The treatments included control (CK), rice straw (R1B0), rice straw biochar prepared at 350℃ (R1B1) and 550℃ (R1B2), rape stalk (R2B0), and rape stalk biochar prepared at 350℃ (R2B1) and 550℃ (R2B2). Straw at 1% and corresponding biochar were added to a strongly acidic red soil. The rice was planted as the experimental crop. Soils were collected at the seedling, tillering, filling and mature stages of rice growth, respectively. The changes in soil pH, exchangeable acidity, organic matter, nutrients (NH₄⁺-N and NO₃^--N), and exchangeable cations in soils were measured. The results showed that soil pH, NH₄⁺-N, and NO₃^--N concentrations decreased with the growth period of rice, while the organic matter content and cation exchange capacity (CEC) increased. Direct returning of straw and biochar could increase soil pH, organic matter content, and exchangeable cations content, and reduce the total amount of exchangeable acids. In the mature stage of rice, rice straw and rape stalk biochar at 350℃ increased the soil pH by 0.29 and 0.42, respectively, compared to the control treatment. Similarly, biochar decreased the exchangeable acidity and exchangeable Al³⁺ content significantly compared to the direct returning treatments of straw. The exchangeable acidity and exchangeable Al³⁺ contents of soils in R1B2 and R2B1 treatments decreased by 54.8% and 58.9%, respectively, compared to the control treatment. The soil organic matter (SOM) content and CEC in biochar treatments were significantly higher than those in direct returning treatments of straw. Overall, the effects of rape stalk biochar on soil properties were slightly stronger than those of rice straw. The correlation analysis showed that soil exchangeable acids had a significantly negative correlation with organic matter (R=-0.912, P<0.01), and CEC (R=-0.866, P<0.05). The CEC in soils was positively related to organic matter (R=0.833, P<0.05). Direct returning of straw and biochar applications can effectively improve soil acidity and increase nutrient contents. The effects of straw biochar on soils were stronger than the direct returning of straw in decreasing soil acidity, and increasing soil organic matter content and exchangeable capacity in acidic soils.

[Effects of nitrogen management on soil microbial community structure at different growth stages under straw returning in paddy soils]

Effects of different nitrogen application methods on microbial community structure of paddy soil at different rice growth stages were examined using phospholipid fatty acid analysis (PLFA) and Biolog technique. There were four treatments, no straw returning or fertilization (CK), straw returning +urea with the proportions of after wheat harvest, before rice transplanting, tillering stage and booting stage being 0:6:2:2 (T₁) and 3:3:2:2 (T₂), and straw returning+co-application of biogas slurry and urea with the proportion of after wheat harvest, before rice transplanting, tillering stage and booting stage being 3 (biogas slurry):3 (2biogas slurry+1urea):2 (urea):2 (urea)(T₃). Results showed that T₃ significantly increased soil available nitrogen contents at all growth stages, which was significantly higher at maturity stage than that at tillering and booting stages. T₁-T₃ had higher available phosphorus and available potassium contents at all growth stages compared with CK, which were higher at tillering stage than at booting and maturity stages. The interaction between growth stage and treatment in paddy soil significantly affected the contents of soil available nitrogen, available phosphorus and available potassium. Furthermore, carbohydrate, amino acid, polymer and carboxylic acid were the primary carbon sources for microbial community of paddy soil. T₃ effectively enhanced soil carbon sources metabolic utilization intensity. The interaction between growth stage and treatment in paddy soil significantly affected the microbial utilization capacity of carbohydrates and carboxylic acids. Soil microbial biomass was significantly higher in T₂ and T₃ treatments. Moreover, T₂ had high fungi/bacteria (F/B) value, indicating that fungi could benefit the stabilization of paddy soil. In summary, simultaneous nitrogen application (urea or biogas slurry) and straw returning could increase soil microbial activity and improve soil environment in paddy field.

[Effects of Fertilizer Reduction and Application of Organic Fertilizer on Soil Nitrogen and Phosphorus Nutrients and Crop Yield in a Purple Soil Sloping Field]

The reduction in chemical fertilizers combined with organic fertilizers is a national strategy to achieve environmental friendliness and maintain the quality of cultivated land. It is of great significance for the prevention and control of soil pollution and the sustainable development of agriculture. In this study, purple soil and sloping land in the Three Gorges Reservoir area was studied. The field experiment method was used to study the control, conventional fertilization, optimized fertilization, biochar (fertilizer combined with biochar), and straw under rapeseed/corn rotation mode. The effects of five treatments on soil nitrogen/phosphorus form, crop nitrogen and phosphorus content, fertilizer utilization rate, and crop yield were studied in the field (fertilizer reduction combined with straw returning). The results showed that the soil ammonium nitrogen content was the highest in the rapeseed season, which was 4.51 mg·kg^-1. The contents of ammonium nitrogen and alkali nitrogen in the treated corn season were significantly higher than those in the rape season. The reduction in chemical fertilizers can guarantee and increase the total nitrogen content of the soil. Among them, the total nitrogen content in the rapeseed and corn seasons treated with straw was the highest (0.56 g·kg^-1 and 0.60 g·kg^-1, respectively). The soil treated with straw in the rapeseed season had the highest available phosphorus content (0.76 mg·kg^-1). Compared with conventional treatment, the reduction of chemical fertilizers combined with organic fertilizer did not significantly reduce the total phosphorus content of soil. The reduction of fertilization combined with organic fertilizer showed a slight increase in yield but showed the highest yield of rapeseed treated by biochar (2328 kg·hm^-2) and the highest yield of conventionally treated maize (5838 kg·hm^-2). However, there was no significant difference in each treatment (P>0.05). Regardless of the rapeseed season or the corn season, the reduction of fertilization treatment generally improved the agronomic utilization rate of nitrogen fertilizer and phosphate fertilizer. In the purple soil area, the combination of chemical fertilizer reduction and biochar and straw returning were beneficial to improve soil nutrients, improve fertilizer utilization, and reduce the effects of nitrogen fertilizer and phosphate fertilizer application on crop yield.

Magnesium accumulation, partitioning and remobilization in spring maize (Zea mays L.) under magnesium supply with straw return in northeast China

BACKGROUND

Magnesium (Mg) has important effects on maize growth, and the application of Mg fertilizer with straw return inevitably has an impact on Mg absorption in maize.

RESULTS

A two-year field trial was conducted to investigate the effects of Mg fertilizers with straw return on Mg accumulations, partitioning and remobilization in maize (Zea mays L.) in northeast China. The treatments included: (i) JM3 (straw + Mg fertilizer), (ii) JM0 (straw + no Mg fertilizer), (iii) WM3 (no straw + Mg fertilizer), and (iv) WM0 (no Mg fertilizer + no straw). The results showed that the highest Mg accumulation stage in maize was prominent between the tasseling stage (VT) and blister stage (R2), and JM3 treatment accumulated 13.3% and 26.6% more Mg on average than those of the WM3 and WM0, respectively. Magnesium remobilization in distinct organs was highest in JM3 and there were significant differences between treatments. The total contribution to the grain for the JM3 treatment was higher by 6.0% and 17.9% on average than those for the WM3 and WM0, respectively. The grain yield of JM3 treatment was 0.5% and 5.3% higher than that of WM3 and WM0, respectively.

CONCLUSION

Generally, these outcomes indicated that there was an interaction between Mg fertilizer and maize straw. The application of Mg fertilizer significantly promoted the accumulation, distribution to the maize organs, and the remobilization of Mg. The combination of straw return and Mg application further increased the accumulation of Mg in the grain. And all these lead to an increase in yield. © 2020 Society of Chemical Industry.

[Effects of irrigation lower limit and straw returning amount on yield, quality and water use efficiency of greenhouse tomato]

A split plot experiment was conducted in greenhouse to investigate the effects of irrigation lower limit and maize straw returning on the yield, quality, and water use efficiency (WUE) of tomato. Maize straw was applied for 1 (2018, A₁), 2(2017, A₂), and 3 (2016, A₃) years at different amounts (0, 1.5×10⁴, 3×10⁴, 4.5×10⁴ kg·hm^-2) and different irrigation lower limits (50%θ_f, 60%θ_f, 70%θ_f, 80%θ_f, θ_f was the field water capacity). Variance analysis, entropy-weight and TOPSIS methods were used to examine the responses of yield, quality and water use efficiency (WUE) of tomato. The results showed that tomato yield was enhanced by the increases of irrigation lower limit. The maximum value of yield was observed in the 80%θ_f treatments, with the maximum average yield of 93.55 t·hm^-2 in A₁, 87.23 t·hm^-2 in A₂, and 99.34 t·hm^-2 in A₃, respectively. WUE and quality of tomato decreased with increasing irrigation lower limit. In the first year of straw returning, the maximum average yield of tomato was 99.60 t·hm^-2 in straw returning 1.5×10⁴ kg·hm^-2 . In the second and third years, 4.5×10⁴ kg·hm^-2 straw returning had the highest average yield of tomato, which was 92.50 and 107.75 t·hm^-2, respectively. The maximum WUE was observed with the straw returning of 1.5×10⁴ kg·hm^-2 in A₁ and A₂, while in the A₃ treatment it happened in straw returning of 4.5×10⁴ kg·hm^-2. The quality index of tomato showed different trends with the increases of straw returning years and amount.

[Effects of Straw Biochar on Soil Microbial Metabolism and Bacterial Community Composition in Drip-irrigated Cotton Field]

A five year field experiment was conducted to evaluate the effect of continually returning cotton straw or biochar on microbial metabolic function and bacterial community composition of soil in a cotton field under drip irrigation conditions. The experiment involved three treatments:control (single application of chemical fertilizer, CK), cotton straw (returning of cotton straw plus chemical fertilizer application, ST), and biochar (returning of cotton straw biochar plus chemical fertilizer application, BC). The returning of cotton straw and biochar both significantly increased soil organic matter, total nitrogen, and available nutrients, but the effect of returning biochar was more significant. The carbon source metabolic activities of the soil in the ST treatment was the highest, followed by the BC treatment, which was significantly higher than of that in the CK treatment. The returning of cotton straw promoted the metabolism of carbohydrate and amine carbon sources, while biochar significantly increased the metabolism of polymer carbon sources. Compared with the CK treatment, the ST treatment significantly increased the phylum of Proteobacteria, Actinobacteria, Bacteroides, and the family of Xanthomonadaceae, Acidobacteriaceae, Microbacteriaceae, and Cytophagaceae. The BC treatment significantly increased the phylum of Acidobacteria, Gemmatimonadetes, Nitrospirae, and the family of Blastocatellaceae (subgroup 4), Gemmatimonadaceae, and Nitrosomonadaceae. The correlation analysis showed that there were significant positive correlations between the relative abundances of Xanthomonadaceae and Acidobacteriaceae and the carbon source metabolic activities of carbohydrates, amino acids, carboxylic acids, and amines. The relative abundances of Microbacteriaceae and Cytophagaceae were positively correlated with carbohydrates and amines. There was a significant positive correlation between the relative abundance of Blastocatellaceae (subgroup 4), Gemmatimonadaceae, Nitrosomonadaceae and the carbon metabolism of polymers. These results suggest that the continual returning of biochar increased soil nutrients, change bacterial community composition, and promoted the metabolic activity of polymer carbon sources in the drip-irrigated cotton field.

[Effects of straw returning combined with nitrogen fertilizer on spring maize yield and soil physicochemical properties under drip irrigation condition in Yellow River pumping irrigation area, Ningxia, China]

Soil compaction and nutrient deficiency are common problems in Ningxia Yellow River pumping irrigation area, which adversely affect crop yield. A two-year (2017-2018) field experiment of straw returning combined with nitrogen fertilizer were designed. Four nitrogen application levels (pure N with 0, 150, 300 and 450 kg·hm^-2) were set under the condition of full smashing of maize straw (12000 kg·hm^-2) returning, with the conventional nitrogen application (pure N with 225 kg·hm^-2) without straw returning as the control (CK) to investigate the effects of straw returning combined with different amounts of nitrogen fertilizer on soil physical and chemical properties and maize yield under drip irrigation condition. The results showed that, compared with no-straw returning treatment, the treatments of straw returning combined nitrogen fertilizer with 300 and 450 kg·hm^-2 reduced soil bulk density (0-20 cm) by 3.3% and 5.4%, but increased soil porosity by 3.7% and 7.1%, respectively. Straw returning combined with nitrogen with 300 kg·hm^-2 and 450 kg·hm^-2 was the best treatment which increased soil organic matter content, available K, P, alkaline N and total N in 0-40 cm soil layer. Compared with the non-returning treatment, straw returning combined with nitrogen fertilizer 300 kg·hm^-2 significantly increased soil water storage by 13.6% and 22.1%, increased maize yield by 31.1% and 46.0 % in 2017 and 2018, respectively. The analysis of yield components showed that the high maize yield was achieved mainly by increasing grain number and the100-grain weight. Curve fitting showed that the optimum amount of nitrogen fertilizer was 260 kg·hm^-2. Our results provide important basis for soil fertility improvement and sustainable production.

[Response of Soil CO2 Emissions to Straw-returning in Citrus/Mushroom Intercropping Systems]

Based on the pattern of citrus tree/stropharia mushrooms intercropping, returning-straw was used as the raw material for the stropharia mushrooms, and an in-situ experiment was conducted to monitor soil CO₂ emissions under different dosage of straw application during the stropharia growth period. Soil CO₂ emissions and the influencing factors were analyzed under different treatments of cultivated (HSM, ASM, and DSM) and uncultivated stropharia mushrooms (HS, AS, and DS). The mushroom yield and soil carbon emission efficiency (CEE) were used to provide a theoretical basis for improving the use of land under citrus orchards. The results showed that:① Straw return increased the cumulative CO₂ emissions compared with the control system (conventional planting, CK) and cumulative CO₂ emissions increased with the dosage of straw application. Cumulative CO₂ emissions from soil treated with cultivated stropharia mushrooms were higher than those from soil treated with uncultivated stropharia mushrooms, in the order of DSM (52.09 t·hm^-2) > ASM (41.10 t·hm^-2) > HSM (33.20 t·hm^-2) > DS (27.15 t·hm^-2) > AS (25.34 t·hm^-2) > HS (18.94 t·hm^-2) > CK (12.16 t·hm^-2). Cumulative CO₂ emissions under the DSM treatment significantly increased by 328.37% compared with CK. ② For the treatment of cultivated stropharia mushrooms, peak soil CO₂ emissions occurred during the period of mycelium growth. The highest cumulative CO₂ emissions during this period were obtained under the DSM treatment and accounted for 43.27% of the total cumulative emissions. This was followed by ASM and HSM which accounted for 42.63% and 40.57% of emissions, respectively. ③ Cultivated stropharia mushrooms reduced the temperature sensitivity coefficient (Q₁₀). The soil temperature (5 cm depth) had a significant effect on the soil CO₂ emission rate (P<0.01) but soil moisture did not (P>0.05). Soil temperature explained 27% to 71% of the variation in soil CO₂ emissions rates, and the two-factor fitting of soil temperature and soil moisture explained 36% to 82% of the variation. ④ For the treatment of cultivated stropharia mushrooms, the ranked yield of each treatment was DSM (49.7 t·hm^-2) > ASM (47.0 t·hm^-2) > HSM (23.3 t·hm^-2), and ASM had the highest soil CEE (1.14). Therefore, under the system of citrus tree/stropharia mushroom intercropping, straw return can increase soil CO₂ emissions, with the highest emissions being obtained when a double dosage of straw was applied. However, the optimal amount of straw still needs to be determined in combination with changes in soil nutrients and crop yields.

[Responses of soil organic carbon and microbial community structure to different tillage patterns and straw returning for multiple years.]

Soil organic carbon is essential for maintaining terrestrial ecosystem function and mitigating soil degradation. Soil microorganisms participate in soil carbon cycling. They are affected by tillage methods and straw returning. A split-plot design was adopted in this experiment. The whole-plot treatment had two tillage methods, subsoil tillage (ST) and rotary tillage (RT). The split-plot treatment included full straw returning (F) and no straw returning (0). The microbial community structure and carbon sequestration genes were assessed by Illumina sequencing technique. Soil organic carbon contents were measured during 2012-2017. The results showed that 1) subsoil tillage and straw returning significantly increased pH, microbial biomass carbon, total nitrogen, silt content, and clay content, while significantly decreased sand content; 2) during the test period (2012-2017), soil organic carbon (SOC) content under all treatments showed an increasing trend, but the increment for average SOC content under straw returning and subsoiling treatments was significantly higher than that of no straw returning and rotary tillage by 33.2 % and 30.6%, respectively; 3) Proteobacteria was the most abundant type of bacteria in the soil, followed by Acidobacteria and Gemmatadanetes; 4) STF treatment maintained high microbial diversity; 5) Excepted for soil sand content, soil pH, microbial biomass carbon, total nitrogen, silt content and clay content all caused the variation of soil microbial community structure under the STF treatment in the direction of SOC accumulation; 6) in addition to the gene abundance in the di- and oligosaccharides metabolic pathway, the gene abundance in the metabolic pathways for CO₂ fixation, central carbohydrate metabolism, fermentation, one-carbon metabolism, organic acids, sugar alcohols and glycoside hydrolases showed that subsoil tillage was significantly higher than rotary tillage, with posi-tively correlation with soil organic carbon content. Therefore, the combination of subsoil tillage and straw returning could improve basic soil properties, affect soil microbial community structure, and increase the capacity of soil carbon fixation, thus providing a realistic basis for solving soil degradation.

[Effects of maize straw returning on arbuscular mycorrhizal fungal community structure in soil]

To understand the effects of agricultural management activities on soil arbuscular mycorrhizal (AM) fungi diversity, the high-throughput sequencing based on Illumina MiSeq platform, and the fatty acids fingerprints were used to examine the effects of maize straw returning on soil arbuscular mycorrhizal fungi. The relationships between AM fungal community composition, AM fungal biomass and soil factors after maize straw returning were examined for four continuous years. A total of 2430 operational taxonomic units (OTUs) of AM fungi were classified into 10 genera and 143 species, respectively, which belonged to 1 phylum, 3 classes, 4 orders, 8 families. There was no significant difference in AM fungal community richness (Chaoles index and ACE index) and diversity (Shannon, Simpson diversity indices) in different treatments. Paraglomus and Glomus were dominant genera among all AM fungal communities. With the increase of the maize straw returned amounts, the abundance of Glomus reduced. Under the treatments of 3000 and 9000 kg·hm^-2 straw returned, the abundance of Glomus and Acaulospora had significant differences with the control (0 kg·hm^-2). Compared with the control, there were significant differences between Archaeospora, Paraglomus and Glomus in the treatment of 3000 kg·hm^-2 straw returned. Results from non-metric multi-dimensional scale (NMDS) analysis showed that under 9000 and 12000 kg·hm^-2 straw returning treatments, the difference between the β diversity of soil AM fungi and the spatial distance of controls was farther apart than the other treatments. The effect of straw returning on the β diversity of AM fungi was significant. The multivariate analysis results revealed the relationship of the spatial variation between the soil physicochemical properties and AM fungi richness and diversity could be explained at 82.8% cumulative variables. The total nitrogen and available nitrogen were the most important factors driving soil microbial communities biomass marked by PLFAs and AM fungal biomass (NLFAs). The continuous maize straw returning to the field changed the genera composition of AM fungi. With the increases of straw returning amounts, the specific species of AM fungi decreased and the similarity between AM fungi community composition decreased. Straw returning increased soil AM fungi biomass and its contribution to soil total microbial biomass.

[Straw Returning Plus Nitrogen Fertilizer Affects the Soil Microbial Community and Organic Carbon Mineralization in Karst Farmland]

The use of straw returning plus nitrogen fertilizer on farmland is one of the important agronomic practices for adjusting soil organic carbon (SOC) transformations. To explore the mechanisms of straw and nitrogen fertilizer application on straw and SOC mineralization in long-term fertilized soils, an incubation experiment with the ¹³C isotope tracing technique was conducted, which involved three long-term fertilized models in typical karst soils (no fertilization, inorganic fertilization, and a combination of inorganic fertilization and straw). To study the mechanisms of ¹³C-labeled straw and SOC mineralization, four treatments were designed as follows:no straw and nitrogen (control), and straw combined with three levels of nitrogen fertilizer (0, 214.0, and 571.0 mg·kg^-1 soil). The results showed that cumulative mineralization amounts of straw-derived organic carbon in long-term fertilized soils were markedly higher than those in non-fertilized soil. Straw-derived organic carbon mineralization was significantly affected by nitrogen fertilizer levels. The positive priming effects (PE) in long-term fertilized soils were much lower than those in non-fertilized soil. The PE was decreased at the low nitrogen fertilizer level but increased at the high nitrogen fertilizer level. The principal component analysis (PCA) of phospholipid fatty acids (PLFAs) indicated that the soil microbial community structure was greatly affected by the long-term fertilization models and combined straw and nitrogen fertilizer application. Moreover, the content of PLFAs in soil microorganisms, namely, bacteria and fungi, were remarkably increased by the straw plus nitrogen fertilizer (values increased by 40.3%-53.0%, 41.1%-62.6%, and 60.5%-148.6% compared with control), but levels were not significantly affected by nitrogen fertilizer levels alone. The ratios between PLFAs of soil gram-positive and gram-negative bacteria (G⁺/G^-) decreased and were stable at around 0.8. The structure equation models (SEM) demonstrated that the combination of straw and nitrogen affected the soil gram-positive and gram-negative bacteria structure and increased the soil DOC content, which promoted the decomposition of straw and affected the mineralization of SOC. These results indicate that straw returning plus low nitrogen fertilizer can improve the SOC sequestration capacity in karst farmland.

[Effects of biochar addition on enzyme activity and fertility in paddy soil after six years]

A field experiment was conducted to examine the effects on soil fertility and enzyme activities in paddy field after six years of one-split rice straw-derived biochar [0 (BC₀), 7.5(BC₁), 15(BC₂), 22.5(BC₃) t·hm^-2] and rice straw (3.75 t·hm^-2, STR) application. The results showed that soil organic carbon, available phosphorus and rapidly available potassium concentrations significantly increased, by 34.6%, 12.4% and 26.2%, respectively. Soil pH and soil bulk density were significantly reduced, but total nitrogen content had no significant difference compared with BC₀. Biochar addition significantly increased the activities of soil urease and acid phosphatase. The soil fluorescein diacetate (FDA hydrolase) and arylsulfatase activity were inhibited to varying degrees. Among them, BC₂ treatment increased soil urease activity by 36.5%. The soil acid phosphatase activity increased with the increases of biochar application rate, which was positively correlated with soil available phosphorus concentration. FDA hydrolase and urease activity had positive correlation with soil available potassium content, while soil acid phosphatase and arylsulfatase activity had positive correlation with soil bulk density. After six years, soil dehydrogenase and polyphenol oxidase activity significantly increased by 48.8% and 27.5%, respectively, while catalase activity significantly decreased when compared with control BC₀. STR treatment increased activities of soil urease, FDA hydrolase, dehydrogenase, acid phosphatase and arylsulfatase significantly, while decreased the catalase and polyphenol oxidase activities by 23.4% and 15.9%, respectively.

[Effects of straw returning amount and type on soil nitrogen and its composition]

Straw returning to soil can supplement soil nutrients required for crop growth, fertilize soil, and improve soil quality. To explore the long-term effect of straw returning on soil total nitrogen and its composition, herein, five treatments including no rice straw + no wheat straw returning (NRW), no rice straw + all wheat straw returning (W), all rice straw + no wheat straw returning (R), half rice straw + half wheat straw returning (HRW), and all rice straw + all wheat straw returning (ARW) were conducted in triplicate in Taihu Lake region, China. The effects of both straw amount and type were examined. Compared with the results obtained in 2007, the results herein obtained in 2017 showed that after 10 years of straw returning, soil total nitrogen and heavy fraction nitrogen increased, while light fraction organic matter decreased. Among the five treatments, ARW had the largest decrease in light fraction nitrogen of 8.09 g·kg^-1; the R treatment had the highest contents of both total and heavy fraction nitrogen, and also the highest contents of ammonium and nitrate. There was no significant difference in alkali-hydrolyzable nitrogen among the five treatments. These results indicated that crop straw was the critical material source for soil nitrogen, and that the effects of straw returning on soil nitrogen depended on the type and amount of crop straw returned to soil. The changes of light fraction nitrogen were more sensitive to straw returning, while the heavy fraction nitrogen was relatively stable, which was the key fraction sustaining soil fertility. With the prolonging of straw returning, the relationship between the total nitrogen and diffe-rent nitrogen components changed. The processing manner of all rice straw returning + no wheat straw returning was the way that could most significantly enhance soil nitrogen content.

High Performance Bacteria Anchored by Nanoclay to Boost Straw Degradation

Generally, crop straw degrades slowly in soil, which is unfavorable for tillage and next crop growth. Thus, it is important to develop a promising technology to boost degradation of straw. Herein, a nanobiosystem has been developed by loading bacterial mixture in nanostructured attapulgite (ATP) and using it as a straw returning agent (SRA). Therein, ATP could effectively anchor bacteria to the surface of straw and greatly facilitate the adhesion and growth of bacteria. Consequently, this technology could effectively accelerate the degradation and transformation of straw into nutrients, including nitrogen (N), phosphorus (P), potassium (K), and organic matters (OM). Pot and field tests indicated that SRA displayed significant positive effects on the growth of the next crop. Importantly, SRA could effectively decrease greenhouse gas emissions from farmland, which is beneficial for the environment. Therefore, this work provides a facile and promising method to facilitate the degradation of straw, which might have a potential application value.

[Effects of stabilized N fertilizer combined with straw returning on rice yield and emission of N2O and CH4 in a paddy field]

Based on a two-year field experiment located at Shenyang Applied Ecological Experiment Station of Chinese Academy of Sciences, we examined the effects of stabilized N fertilizer combined with straw returning on rice yield and emission of N₂O and CH₄ in aquic brown soil. Six treatments were set up, i.e. control (CK), urea(U), urea+urease inhibitor+nitrification inhibitor (U+I), straw (S), straw+urea (S+U), straw+urea+ urease inhibitor+nitrification inhibitor (S+U+I). The results showed that urea application increased rice yield, cumulative N₂O and CH₄ emission, and global warming potential. The treatment of U+I significantly mitigated cumulative N₂O emission. Returning rice straw to the field significantly increased cumulative N₂O emission, cumulative CH₄ emission, global warming potential, and greenhouse gas emission intensity. The S+U+I treatment had the highest rice yield and greenhouse gas emission intensity. U+I treatment had the the second highest rice yield and the lowest greenhouse gas emission intensity. Rice yield in the S treatment showed no significant difference with CK. Our results indicated that S+U+I and U+I are relatively better agricultural strategies compared with other treatments in paddy fields on aquic soil.

[Effects of subsoiling and straw returning on soil physical properties and maize production in Yellow River irrigation area of Gansu, China]

A field experiment was conducted to examine the effects of subsoiling 35 cm with maize straw returning, subsoiling 35 cm without maize straw returning, and rotary tillage without maize straw returning on soil compaction, soil bulk density, soil infiltration, soil water content in 0-100 cm depth, nutrients uptake and production of maize on sierozem in the Gansu Yellow River irrigated area in 2015-2017. Compared with subsoiling 35 cm without maize straw returning and rotary tillage without maize straw returning, subsoiling 35 cm with maize straw returning significantly decreased the soil compaction and soil density in 0-40 cm depth. Compared with that in 2015 (before experiment), soil compaction and soil bulk density in subsoiling 35 cm with straw returning was decreased by 42.6% and 7.0%, respectively, after harvest in 2017. Compared with other treatments, subsoiling 35 cm with straw returning had the lowest variation of soil compaction (6.1%) and soil bulk density (3.2%) in 0-40 cm depth before sowing and after harvest in 2016 and 2017. The soil infiltration rate in subsoiling 35 cm with straw returning was significantly improved by 33.6% compared with rotary tillage without maize straw returning. Subsoiling 35 cm with straw retention could significantly increase soil water content and decrease water variation in 0-100 cm soil depth in spring (before maize sowing) and autumn (after maize harvest). Compared with rotary tillage without maize straw returning, water storage in subsoiling 35 cm with straw retention was increased by 15.5% and 5.6% in spring and autumn, respectively. The water use efficiency was enhanced by 32.4%. Furthermore, subsoiling 35 cm with straw retention could increase maize economic yield and biomass yield by 25.6% and 33.3%, compared with rotary tillage without straw retention. Subsoilng and straw retention could promote nutrient absorption, with N, P₂O₅ and K₂O uptake increased by 49.6%, 51.5% and 37.6%, compared with rotary tillage. Overall, our results suggested that subsoiling 35 cm straw retention could improve soil characteristics, stabilize the phy-sical properties of the plough layer, increase soil water content in the 0-100 cm soil layer, and reduce water variation in spring and autumn. Consequently, it was the best management to promote the water and nutrient utilization of maize and achieve high yield. Our findings could provide theoretical basis for further research on the construction technology of the plough layer in Gansu irrigation area.

[Effects of sewage irrigation on growth of rice seedlings and soil environment with straw incorporation.]

The effects of sewage irrigation on the growth of rice seedlings and soil environment under wheat straw returning were examined with a pot experiment. Root morphology, root activity, tiller number, plant height, dry matter accumulation of rice seedling, soil ferrous ions content, organic acid content and enzyme activity were measured. The results showed that sewage irrigation significantly increased the number of tillers and root activity at 41 days after transplanting under no N fertilizer application. Under the same N input level, sewage irrigation combined with N fertilizer promoted the growth of rice seedlings and root, and increased the root length, root surface area, root volume, root activity, tiller number and dry matter accumulation. Sewage irrigation significantly reduced the contents of soil ferrous ions and organic acid, while significantly increased the activities of soil urease and catalase. These results indicated that the combination of sewage irrigation and N fertilizer could effectively reduce the negative effect of straw returning on rice seedling and thus enhance soil fertility and quality.

[Effects of returning straw with nitrogen application on soil water and nutrient status, and yield of maize]

The study focused on the problems of lean soil and low fertility in arid area of central Ningxia. To explore the optimum rate of nitrogen (N) fertilizer application under straw returning, taking the treatment with straw return and no nitrogen fertilization as control, we investigated the effects of three N application levels (150, 300, 450 kg·hm^-2) with return of total maize straw (9000 kg·hm^-2) on soil water and nutrient status and maize yield. The results showed that the soil water storage (0-100 cm) at 300 and 450 kg N·hm^-2 in middle and late growing period of maize was significantly increased by 10.1% and 9.0%. The enhancement of soil fertility was highest at 300 kg N·hm^-2, with significant increases of the contents of soil organic matter, total N, alkali-hydrolyzable-N, available P, and available K by 12.8%, 31.6%, 11.6%, 20.6% and 74.2%, respectively. The enhancement of maize grain yield was highest at 300 and 450 kg N·hm^-2, with the value being 32.1% and 23.7%, respectively. The net income at 300 and 450 kg·hm^-2 N was significantly increased by 31.8% and 16.8%, respectively. Our results showed that straw returning plus proper quantity of N fertilizer could improve soil water and fertilizer status and enhance maize yield and net income in arid area of central Ningxia. The treatment of straw returning with 300 kg·hm^-2 N was the best one.

[Effects of reducing N, straw returning and dicyandiamide application on winter wheat yield and nitrogen budgets in rain-fed region.]

In a 3-year field experiment, effects of reducing N fertilizer, straw returning and dicyandiamide (DCD) application on wheat yield and nitrogen budgets under wheat-fallow system were studied in rain-fed region. The experiment was carried out in Yangling of Shaanxi Province, and 4 treatments were installed including no nitrogen (CK), 220 kg N·hm^-2 and no straw returning (FP), 150 kg N·hm^-2 and straw returning (OPT), 150 kg N·hm^-2, 7.5 kg·hm^-2 DCD and straw returning (OPT+DCD). The results showed there was no remarkable difference for wheat yield between OPT and FP, but the N use efficiency of the former was 6.1% more than that of the latter, and the apparent N loss ratio was 7.2% less. OPT+DCD increased the average yield of winter wheat by 10.4% and 7.9% respectively compared to OPT and FP, the N use efficiency of winter wheat increased by 20.8% and 28.1%, and the apparent N loss ratio decreased by 8.5% and 15.1%, respectively. NH₄⁺-N content in 0-20 cm of soil increased, accordingly the NO₃^--N content decreased in 40 to 45 days when DCD was applied.

[Response of Soil Respiration and Heterotrophic Respiration to Returning of Straw and Biochar in Rape-Maize Rotation Systems]

Soil respiration has become the main way of farmland ecosystem carbon emissions. Soil respiration and its responses to soil moisture and soil temperature under straw and biochar returning were investigated. Combined soil CO₂ fluxes system(ACE-002/OPZ/SC) with the method of root exclusion, this study conducted a long-term field experiment in the national monitor station of soil fertility and fertilizer efficiency of purple soils. The total soil respiration and heterotrophic respiration rate and the soil hydrothermal factors were measured during the growth period of rape and maize in rape-maize rotation systems, and the difference between total soil respiration and heterotrophic respiration was calculated as the contribution of root respiration to soil respiration. There were five treatments including CK(no organic material), CS(straw), CSD(straw+microorganism), BC(biochar), CSBC(50%straw+50%biochar), which were replicated three times. The results showed that straw and biochar returning significantly affected the seasonal variations and the peak of soil respiration. In addition to BC treatment, other treatments promoted soil respiration and cumulative emissions of soil CO₂. Soil respiration rate was significantly different under different treatments, the changes in soil respiration rates showed a single peak curve under all treatments, the seasonal variations in soil respiration rates under rape was 0.12-2.29 μmol·(m²·s)^-1, displaying an order of CS > CSD > CSBC > CK > BC. Soil respiration was pretty complex in maize season, the seasonal variation in soil respiration rates under rape was 1.02-15.32 μmol·(m²·s)^-1, displaying an order of CSD > CS > CSBC > CK > BC, the changes in soil respiration rate presented a double peak curve under CS and CSD and CSBC treatments and a single peak curve under BC and CK treatments. Heterotrophic respiration could explain 86.50%-93.94% of seasonal variations in the soil total respiration, and the contribution of root respiration(26.49%-32.86%) was significantly lower than CK treatment(53.65%).Straw and biochar returning did not change soil temperature and soil moisture. Soil temperature at 5 cm depth had significant effects on the change dynamics of soil respiration rates, but soil moisture did not. Soil temperature at 5 cm depth could explain 82%-94% of the variations in soil respiration. The values of temperature sensitivity coefficient changed from 3.28 to 4.47. Compared with CK treatment, Q₁₀ of CS, CSD and CSBC decreased by 26.62%, 18.12%, 20.58%, respectively, while BC increased by 12.53%. There was no synergistic effect between soil temperature and soil moisture on soil respiration, the dynamic changes of soil respiration rate could be simulated by single factor index function of soil temperature. Overall, soil respiration was significantly promoted by returning of straw, straw+microorganism, straw+biochar, while it was inhibited by returning of biochar.

[Effects of Straw in Combination with Reducing Fertilization Rate on Soil Nutrients and Enzyme Activity in the Paddy-Vegetable Rotation Soils]

The effects of straw returning combined the reducing application of chemical fertilizer for crop yield, soil nutrients and enzyme activity were studied in a typical southwestern hilly area of China in a rice-vegetable (Brassica juncea var. gemmifera Lin.) cropping system. The purple soil was selected as the target type of soil, and the Pioneer Town, Jiangjin District, Chongqing, China was selected as the typical southwestern hilly area during 2013 and 2014.Scientific basis based on the optimized fertilization strategy and the recyclable utilization of straw was provided through a field in-situ experiment in this study. There were five treatments in the field experiment: F (conventional fertilizer), 90% F+AS (100% straw with 90% conventional fertilizer), 80% F+AS (100% straw with 80% conventional fertilizer), 70% F+AS (100% straw with 70% conventional fertilizer), 50% F+DS (200% straw with 50% conventional fertilizer), and the 100% straw was 7500 kg·hm^-2. The results showed that the yields of rice and vegetable were all increased in straw with reduced fertilizer treatments (3.0%-17.9% in rice yield and 12.2%-36.4% in vegetable yield) compared with conventional fertilizer (F) treatment. Moreover, the yield of rice in the second season was also increased by 820-1240 kg·hm^-2. Soil pH values in straw with reduced fertilizer treatments raised by 0.06-0.55 compared with F treatment, especially in straw with 70% and 80% of conventional fertilizer dosage which increased by 6.74-6.88 and 6.52-6.84, respectively. The highest content of soil organic matter was 41.01 g·kg^-1 in straw with 80% of conventional fertilizer treatment. For the aspect of soil available nutrients, straw with 80% of conventional fertilizer treatment increased 110-178 mg·kg^-1 content of soil available nitrogen and 31.3-64.0 mg·kg^-1 content of soil available phosphorus. However, the excessive application of straw had negative effect on the accumulation of soil available phosphorus. In the same cultivation period, the contents of soil urease increased significantly in straw with 70% and 80% of conventional fertilizer treatments, which increased by 13.6%-76.4% and 20.1%-75.0% compared with F treatment. The contents of soil catalase in straw with reduced fertilizer treatments were significantly higher than F treatment; in first two seasons, soil catalase contents increased by 0.37 and 0.31 mL·(h·g)^-1 relative to F treatment in straw with reduced fertilizer treatment (80% of conventional fertilizer). With the increasing time of planting, the soil phosphatase content in the third season was higher than those in first two seasons. Straw with 70% and 80% of fertilizer treatments increased the activity of soil phosphatase by 45.2% and 48.2% compared with F treatment. It was concluded that straw application with 70% or 80% dosage of conventional fertilizer could benefit the rice-vegetable rotation in the southwest hilly area of China.

[Effect of Straw Incorporation and Domestic Sewage Irrigation on Ammonia Volatilization from Paddy Fields]

A pot experiment was conducted to study the effect of straw returning and domestic sewage irrigation on the dynamics of NH₄⁺-N concentration and pH in the flood water, and ammonia volatilization of paddy fields. The results showed that the NH₄⁺-N concentration in flood water was significantly increased by wheat straw returning while significantly decreased by domestic sewage irrigation. The cumulative ammonia volatilization in the whole rice season under tap water irrigation and straw removal treatment was 58.29 kg·hm^-2, accounting for 24.29% of the total N applied. The N loss ratio of ammonia volatilization was significantly increased to 45.66% by wheat straw returning, while significantly decreased to 17.26% under straw removal and 32.72% under straw returning by domestic sewage irrigation. Significant positive interaction was observed between straw incorporation and domestic sewage irrigation on ammonia volatilization loss. The average N loss from ammonia volatilization during the tillering stage was the highest among the three fertilization stages, accounting for 7.38%-24.44% of the total N applied. In addition, ammonia volatilization fluxes showed a significant positive correlation with the flood water NH₄⁺-N concentration, irrespective of the irrigation water, but had no significant correlation with pH. These results indicated that straw returning increased ammonia volatilization losses, whereas domestic sewage irrigation could effectively reduce ammonia volatilization losses and simultaneously replace 44.41% of chemical nitrogen fertilizer by the N contained in the domestic sewage. The combination of domestic sewage irrigation and straw returning would be an ecological and environmental-friendly measure for rice nitrogen management in Taihu Lake region.

[Effects of straw returning combined with medium and microelements application on soil organic carbon sequestration in cropland.]

A 52-day incubation experiment was conducted to investigate the effects of maize straw decomposition with combined medium element (S) and microelements (Fe and Zn) application on arable soil organic carbon sequestration. During the straw decomposition, the soil microbial biomass carbon (MBC) content and CO₂-C mineralization rate increased with the addition of S, Fe and Zn, respectively. Also, the cumulative CO₂-C efflux after 52-day laboratory incubation significantly increased in the treatments with S, or Fe, or Zn addition, while there was no significant reduction of soil organic carbon content in the treatments. In addition, Fe or Zn application increased the inert C pools and their proportion, and apparent balance of soil organic carbon, indicating a promoting effect of Fe or Zn addition on soil organic carbon sequestration. In contrast, S addition decreased the proportion of inert C pools and apparent balance of soil organic carbon, indicating an adverse effect of S addition on soil organic carbon sequestration. The results suggested that when nitrogen and phosphorus fertilizers were applied, inclusion of S, or Fe, or Zn in straw incorporation could promote soil organic carbon mineralization process, while organic carbon sequestration was favored by Fe or Zn addition, but not by S addition.