Response of soil nutrients retention and rice growth to biochar in straw returning paddy fields

Effects of organic material addition on carbon cycling and soil fertility in paddy soil

Soil organic carbon (SOC) plays a crucial role in plant nutrient supply and soil physical, chemical, and biological function regulation. Factors such as climate change, human activities, and farm management practices can adversely affect SOC. Here, paddy soil in the double-cropping rice area of Hunan Province of China was cultured with eight kinds of organic materials, and the effects of organic fertilization on SOC content, humus content, and soil carbon sequestration efficiency were analyzed. The addition of straw, green fertilizer, and organic fertilizer had a positive influence on active SOC content. Straw addition had the most prolonged impact on soil microbial biomass carbon content, and green manure had the most rapid influence on soil dissolved organic carbon content. All organic materials enhanced the soil humus content; furthermore, the addition of organic fertilizers significantly improved the carbon sequestration efficiency among all treatments. The results of controlled culture experiments in paddy soils established that applying organic materials can increase SOC content, active carbon components, soil humus content, and the carbon fixation rate, thereby improving soil fertility. This study provides a theoretical basis for the optimal fertilization of paddy soil with organic material addition in China.

Impact of straw returning on soil ecology and crop yield: A review

Several studies have demonstrated the effect of straw return on enhancing soil ecology, promoting sustainable agricultural practices, and cumulative effects on plant yield. Recent studies have focused on straw return methods and their impact on soil nutrient cycling and the overall physicochemical composition of the soil. Despite the substantial progress and successes, several research gaps in these studies require further investigations to harness the full potential of straw return. This review provides a thorough examination of straw diversity and decomposition mechanisms, the effects of straw on soil microorganisms, the interactions between cellulolytic nitrogen-fixing microbes and lignocellulose biomass, as well as nutrient mineralization, organic matter content, and their influence on plant growth and yield. This review also examined the effects of straw return on plant pathogens and its allelopathic impact on plant growth, highlighting research gaps to encourage further studies that could fully realize the potential benefits of straw return in agricultural fields for optimal plant growth.

Effectiveness of constructed wetland technology-treated industrial wastewater on the spinach (Spinacia oleracea) health risks and biochar efficiency

In peri-urban areas, use of industrial wastewater for irrigation is a common practice. Industrial wastewater contains cadmium, chromium, lead, nickel, and other elements that deteriorate food quality and affect human health. Biochar has been proven to remediate heavy metal contaminated soil by reducing their mobility and bioavailability. A pot experiment was conducted to evaluate the efficiency of different levels of biochar on spinach growth with low heavy metal concentration and to minimize associated health issues. The experiment lasted two months and the treatments: Control (tap water), untreated and treated industrial wastewater and both in combination with biochar (0.5% and 1%) were applied in completely randomized design. Findings suggested that treated industrial wastewater with 1% biochar resulted in maximum plant height, shoot weight, chlorophyll contents (SPAD value), photosynthetic and transpiration rate. Biochar significantly reduced heavy metal mobility in soil due to its porous structure, high pH, higher CEC, and variety of surface functional groups. The cumulative hazard index (HI), hazard quotient, cancer risk, and total cancer risk (TCR) were calculated using method provided by US-EPA for each metal. All treatments had HI values of < 1, however applying 1% biochar significantly reduced the HI values to 2.00E-01 and 2.88E-01 in adults and children, respectively. TCR for all treatments was < 1, while treated industrial wastewater and biochar (1%) has significantly reduced to 1.55E-02 and 1.91E-03 for adults and children, respectively. Thus, it was determined that irrigation with industrial effluents caused toxicity in vegetables, which had a negative impact on human health. Biochar effectively mitigated metal toxicity in both soil and spinach plants that resulted in reduced health/cancer risk.

Novel nanocomposite and biochar insights to boost rice growth and alleviation of Cd toxicity

Cadmium (Cd) is an unessential and pervasive contaminant in agricultural soil, eventually affecting the food and instigating health issues. The implication of nanocomposites in agriculture attained significant attention to drive food security. Nanocomposites possess exceptional characteristics to stun the challenges of chemical fertilizers that can enhance plant yield and better nutrient bioavailability. Similarly, biochar has the ability to immobilize Cd in soil by reducing mobility and bioavailability. Rice husk biochar is produced at high temperature pyrolysis under anoxic conditions and a stable carbon-rich material is formed. To strive against this issue, rice plants were subjected to Cd (15, 20 mg kg- 1) stress and treated with alone/combined Ca + Mg (25 mg L- 1) nanocomposite and rice husk biochar. In our study, growth and yield traits showed the nurturing influence of Ca + Mg nanocomposite and biochar to improve rice defence mechanism by reducing Cd stress. Growth parameters root length 28%, shoot length 34%, root fresh weight 19%, shoot fresh weight 16%, root dry weight 9%, shoot dry weight 8%, number of tillers 32%, number of grains 20%, and spike length 17% were improved with combined application of Ca + Mg and biochar, with Cd (20 mg kg- 1), rivalled to alone biochar. Combined Ca + Mg and biochar application increased the SPAD 23%, total chlorophyll 26%, a 19%, b 18%, and carotenoids 15%, with Cd (20 mg kg- 1), rivalled to alone biochar. MDA 15%, H2O2 13%, and EL 10% were significantly regulated in shoots with combined Ca + Mg and biochar application with Cd (20 mg kg- 1) compared to alone biochar. POD 22%, SOD 17%, APX 18%, and CAT 9% were increased in shoots with combined Ca + Mg and biochar application with Cd (20 mg kg- 1) compared to alone biochar. Cd uptake in roots 13%, shoots 14%, and grains 21% were minimized under Cd (20 mg kg- 1) with combined Ca + Mg and B. pumilus application, compared to alone biochar. Subsequently, combined Ca + Mg and biochar application is a sustainable solution to boost crop production under Cd stress.

Evaluation of water management on arsenic methylation and volatilization in arsenic-contaminated soils strengthened by bioaugmentation and biostimulation

Arsenic (As) fate in paddy fields has been one of the most significant current issues due to the strong As accumulation potential of rice plants under flooded conditions. However, no attempt was done to explore As methylation and volatilization under non-flooded conditions. Herein, we investigated the effects of water management on As methylation and volatilization in three arsenic-contaminated soils enhanced by biostimulation with straw-derived organic matter and bioaugmentation with genetic engineered Pseudomonas putida KT2440 (GE P. putida). Under flooded conditions, the application of biochar (BC), rice straw (RS) and their combination (BC+RS) increased total As in porewater. However, these effects were greatly attenuated under non-flooded conditions. Compared with RS amendment alone, the combination of GE P. putida and RS further promoted the As methylation and volatilization, and the promotion percentage under non-flooded conditions were significantly higher than that under flooded conditions. The combined GE P. putida and RS showed the highest efficiency in As methylation (88 µg/L) and volatilization (415.4 µg/(kg·year)) in the non-flooded soil with moderate As contamination. Finally, stepwise multiple linear regression analysis presented that methylated As, DOC and pH in porewater were the most important factors contributing to As volatilization. Overall, our findings suggest that combination of bioaugmentation with GE P. putida and biostimulation with RS/BC+RS is a potential strategy for bioremediation of arsenic-contaminated soils by enhancing As methylation and volatilization under non-flooded conditions.

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.

The use of urea hydrogen peroxide as an alternative N-fertilizer to reduce accumulation of arsenic in rice grains

A greenhouse experiment was conducted to examine the effects of urea hydrogen peroxide (UHP) on reducing the accumulation of As in rice grains. The results show that UHP effectively triggered Fenton-like reaction by reacting with Fe2+ in the paddy soils. This significantly inhibited the activities of As(V)-reducing microbes, causing impediment of As(V)-As(III) conversion following inundation of dryland crop soils for paddy rice cultivation. As-methylating microbes were also inhibited, adversely affecting As methylation in the soils. These processes led to the reduction in phyto-availability of As in the soil solutions for uptake by rice plant roots, and consequently reduced the accumulation of As in the rice grains. In this study, an UHP application rate of 0.0625% on three occasions (tillering, heading and filling) during the rice growth period was sufficient to lower the rice grain-borne As concentration to below 0.2 mg/kg, meeting the quality standard set by the Chinese government. No additive effect on reducing grain-borne As was observed for the joint application of UHP and biochar or biochar composite. The use of UHP for soil fertilization had no adverse impact on rice yield in comparison with the application of urea at an equal amount of nitrogen.

Mechanisms underlying soil microbial regulation of available phosphorus in a temperate forest exposed to long-term nitrogen addition

With exogenous nitrogen (N) input into soil, phosphorus (P) could become a limiting nutrient for plant growth. Soil microbes play a crucial role in regulating soil P cycle and availability. P functional genes, further, regulate soil P availability. It is unclear how the addition of N in different chemical forms and rates influences the composition of soil microbes associated with P cycling and the abundance of P functional genes. A long-term experiment of N addition in three chemical forms with two levels in a temperate forest was performed to reveal the influences and the underlying mechanisms. We found that both chemical N forms and N rates selected for different P-solubilizing microbes. Ammonia form-N increased the abundances of P-solubilizing bacteria at low and high rates. Continuous N deposition included a significant decrease in soil pH and inhibited the viability and activity of bacterial communities in soil, especially the P-solubilizing bacteria. Thus, it restricted inorganic P mobilization and led to a decrease in soil available P. In addition, ammonium-N enhanced the relative abundance of most of the functional genes related to organic P mineralization, while nitrate-N presented a decrease trend. Ammonium-N significantly decreased most of the functional genes relevant to P transportation, whereas the other chemical N forms did not change them. Although N-addition consistently decreased the functional genes relevant to inorganic P solubilization, two of them (ppx and ppa) were the exceptions and showed an increase trend. N addition also decreased soil pH and altered soil properties, and indirectly contributed to the changes in community composition of P-solubilizing microbes and the abundances of multiple P functional genes. Our results provide a mechanistic explanation for the regulation of microbes on N-induced available P limitation via tuning the compositions of P-solubilizing microbes and the abundances of multiple P functional genes.

Improved sea rice yield and accelerated di-2-ethylhexyl phthalate (DEHP) degradation by straw carbonization returning in coastal saline soils

Di-2-ethylhexyl phthalate, a persistent organic contaminant, is widely distributed in the environment and poses substantial threats to human health; however, there have been few investigations regarding the risks and remediation of DEHP in coastal saline soils. In this work, we studied the influences of straw carbonization returning on sea rice yield and DEHP degradation. Straw carbonization returning significantly increased soil nutrients and reduced salt stress to improve sea rice yield. DEHP degradation efficiency was enhanced to a maximum of 78.27% in straw carbonized return with 60% sea rice, mainly attributed to the high pH value, high soil organic matter and enriched potential DEHP degraders of Nocardioides, Mycobacterium and Bradyrhizobium. Some key genes related to metabolism (esterase and cytochrome P450) and DEHP-degradation (pht4, pht5, pcaG, dmpB, catA and fadA) were elevated and explained the accelerated DEHP degradation, shifting from the benzoic acid pathway to the protocatechuate pathway in straw carbonization returning. The results obtained in this study provide a deep and comprehensive understanding of sea rice yield improvement and DEHP degradation mechanisms in coastal paddy soil by a straw carbonization returning strategy.