[Effects of seed grading on population regularity degree and yield of summer maize]

Grading seeds based on grain size is an effective measure to improve population regularity degree and increase the yield of summer maize. Taking Denghai 605 as the experimental material, we set up a field experiment with treatments based on grain size: large seeds (L), medium-round seeds (MR), medium-flat seeds (MF), medium-round and medium-flat mixed seeds (MRF), and small seeds (S), with no-grading seeds as control (CK). We investigated seedling emergence rate, population regularity degree (including height, ear height and stem diameter), seedling sturdiness index, photosynthetic characteristics, dry matter accumulation and distribution characteristics, and yield. The results showed that the emergence rate followed an order of L>MR>MRF>MF>CK>S, with that of L treatment differed little from MR, MF and MRF treatments, but being significantly higher than S and CK treatments. Plant height and stem diameter population regularity degree of MRF treatment before seven-leaf stage was not different from those of L, MR, MF and S treatments, but significantly higher than those of CK. At the tasseling stage, all treatments had higher population regularity degree of plant height than other stages. Ear height population regularity degree of L, MR, MF, MRF, and S increased by 11.1%, 10.3%, 9.5%, 7.1%, and 6.4% compared with CK, respectively. The seedling sturdiness index of MRF treatment increased by 36.7% compared with S treatment, but was not significantly different from L treatment. The leaf area index of the L and MRF treatments was significantly higher than that of CK, and both had higher population photosynthetic properties. The population dry matter accumulation showed a pattern as L>MR>MRF>MF>CK>S. There was no significant difference among L, MR, and MRF treatments, but that in L being obviously higher than MF, CK, and S treatments. After seed grading, the number of harvested ears of the L and MRF treatments increased significantly, and the yield were shown as L>MR>MRF>MF>CK>S. There was no difference between the yield of MRF, MR and MF treatments. In conclusion, the performance of L treatment was improved but the number was small. Considering the grading cost and yield, the MRF treatment can save the seed amounts of sowing, realize mechanized sowing and precision sowing.

Effects of Salt Tolerance Training on Multidimensional Root Distribution and Root-Shoot Characteristics of Summer Maize under Brackish Water Irrigation

To investigate the impact of brackish water irrigation on the multidimensional root distribution and root-shoot characteristics of summer maize under different salt-tolerance-training modes, a micro-plot experiment was conducted from June to October in 2022 at the experimental station in Hohai University, China. Freshwater irrigation was used as the control (CK), and different concentrations of brackish water (S0: 0.08 g·L-1, S1: 2.0 g·L-1, S2: 4.0 g·L-1, S3: 6.0 g·L-1) were irrigated at six-leaf stage, ten-leaf stage, and tasseling stage, constituting different salt tolerance training modes, referred to as S0-2-3, S0-3-3, S1-2-3, S1-3-3, S2-2-3, and S2-3-3. The results showed that although their fine root length density (FRLD) increased, the S0-2-3 and S0-3-3 treatments reduced the limit of root extension in the horizontal direction, causing the roots to be mainly distributed near the plants. This resulted in decreased leaf area and biomass accumulation, ultimately leading to significant yield reduction. Additionally, the S2-2-3 and S2-3-3 treatments stimulated the adaptive mechanism of maize roots, resulting in boosted fine root growth to increase the FRLD and develop into deeper soil layers. However, due to the prolonged exposure to a high level of salinity, their roots below 30 cm depth senesced prematurely, leading to an inhibition in shoot growth and also resulting in yield reduction of 10.99% and 11.75%, compared to CK, respectively. Furthermore, the S1-2-3 and S1-3-3 treatments produced more reasonable distributions of FRLD, which did not boost fine root growth but established fewer weak areas (FLRD < 0.66 cm-3) in their root systems. Moreover, the S1-2-3 treatment contributed to increasing leaf development and biomass accumulation, compared to CK, whereas it allowed for minimizing yield reduction. Therefore, our study proposed the S1-2-3 treatment as the recommended training mode for summer maize while utilizing brackish water resources.

Nitrogen reduction combined with ETc irrigation maintained summer maize yield and increased water and nitrogen use efficiency

Introduction

High rainfall and excessive urea application are counterproductive to summer maize growth requirements and lower grain yield and water/nitrogen (N) use efficiency. The objective of this study was to determine whether ETc irrigation based on summer maize demand and reduced nitrogen rate in the Huang Huai Hai Plain increased water and nitrogen use efficiency without sacrificing yield.

Methods

To achieve this, we conducted an experiment with four irrigation levels [ambient rainfall (I0) and 50% (I1), 75% (I2), and 100% (I3) of actual crop evapotranspiration (ET_c)] and four nitrogen rates [no nitrogen fertilizer (N0), recommended nitrogen rate of urea (NU), recommended nitrogen rate of blending controlled-release urea with conventional urea fertilizer (BCRF) (NC), and reduced nitrogen rate of BCRF (NR)] in 2016-2018.

Results

The results show that reduced irrigation and nitrogen rate reduced Fv/Fm, ¹³C-photosynthate, and nitrogen accumulation both in the kernel and plant. I3NC and I3NU accumulated higher ¹³C-photosynthate, nitrogen, and dry matter. However, ¹³C-photosynthate and nitrogen distribution to the kernel was decreased from I2 to I3 and was higher in BCRF than in urea. I2NC and I2NR promoted their distribution to the kernel, resulting in a higher harvest index. Compared with I3NU, I2NR increased root length density by 32.8% on average, maintaining considerable leaf Fv/Fm and obtaining similar kernel number and kernel weight. The higher root length density of I2NR of 40-60 cm promoted ¹³C-photosynthate and nitrogen distribution to the kernel and increased the harvest index. As a result, the water use efficiency (WUE) and nitrogen agronomic use efficiency (NAUE) in I2NR increased by 20.5%-31.9% and 11.0%-38.0% than that in I3NU, respectively.

Discussion

Therefore, 75%ET_c deficit irrigation and BCRF fertilizer with 80% nitrogen rate improved root length density, maintained leaf Fv/Fm in the milking stage, promoted 13C-photosynthate, and distributed nitrogen to the kernel, ultimately providing a higher WUE and NAUE without significantly reducing grain yield.

Late Split-Application with Reduced Nitrogen Fertilizer Increases Yield by Mediating Source-Sink Relations during the Grain Filling Stage in Summer Maize

In the North China Plain, the excessive application of nitrogen (N) fertilizer for ensuring high yield and a single application at sowing for simplifying management in farmer practice lead to low N use efficiency and environmental risk in maize (Zea mays L.) production. However, it is unclear whether and how late split application with a lower level of N fertilizer influences maize yield. To address this question, a two-year field experiment was conducted with two commercial maize cultivars (Zhengdan 958 and Denghai 605) using a lower level of N input (180 kg ha^-1) by setting up single application at sowing and split application at sowing and later stages (V12, R1, and R2) with four different ratios, respectively. The maize yield with split-applied 180 kg ha^-1 N did not decrease compared to the average yield with 240 kg ha^-1 N input in farmer practice, while it increased by 6.7% to 11.5% in the four N split-application treatments compared with that of the single-application control. Morphological and physiological analyses demonstrated that late split application of N (i) increased the net photosynthetic rate and chlorophyll content and thus promoted the photosynthetic efficiency during the reproductive stages; (ii) promoted the sink capacity via improved kernel number, endosperm cells division, and grain-filling rate; and (iii) increased the final N content and N efficiency in the plant. Therefore, we propose that late split application of N could reduce N fertilizer input and coordinately improve N efficiency and grain yield in summer maize production, which are likely achieved by optimizing the source-sink relations during the grain-filling stage.

Effects of agronomic traits and climatic factors on yield and yield stability of summer maize (Zea mays L) in the Huang-Huai-Hai Plain in China

Zhengdan 958 (ZD958) is the summer maize variety with the widest planting area in Huang-Huai-Hai plain in the past 20 years. Understanding the agronomic characteristics of maize and its adaptability to climatic factors is of great significance for breeding maize varieties with high yield and stability. In this study, the experimental data of 33 experimental stations from 2005 to 2015 were analyzed to clarify the effects of different agronomic traits on yield and the correlation between agronomic traits, and to understand the effects of different climatic factors on summer maize yield and agronomic traits. The results showed that the average yield of ZD958 was 9.20 t ha^-1, and the yield variation coefficient was 13.41%. There was a certainly negative correlation between high yield and high stability. Plant heights, ear heights, double ear rate, ear length, ear rows, line grain number, grain number per ear, ear diameter, cob diameter, and 1000 grains weight were significantly positive correlation with maize yield. Solar radiation before and after silking were significantly positive correlation with maize yield. Path analysis showed that changes in agronomic traits accounted for 54% of the yield variation, and changes in climate factors accounted for 26% of the yield variation. Our study showed that higher plant height, ear height, grain number per ear and 1000-grain weight, lower lodging rate, pour the discount rate and shorter bald tip long were the main reasons for high yield. Among the climatic factors, solar radiation and the lowest temperature have significant effects on the yield.

Effects of hydrogen peroxide priming on yield, photosynthetic capacity and chlorophyll fluorescence of waterlogged summer maize

Extreme rainfall events during the summer maize growth and development periods, which have induced losses in summer maize production. There was a completely randomized block experiment being designed with four treatments: waterlogging for 6 days at the V3 stage (C-W), H₂O₂-priming + non-waterlogging (H-CK), H₂O₂-priming + waterlogging for 6 days at the V3 stage (H-W) and control (C-CK). This study investigated the effects of H₂O₂ priming on yield and photosynthetic parameters of (Zea mays. L) summer maize hybrid DengHai605 (DH605) by measuring the leaf area index (LAI), soil and plant analyzer development (SPAD) value, stomatal morphology, gas exchange parameters, and chlorophyll fluorescence parameters. The results showed that the net photosynthetic rate (Pn) was decreased after waterlogging through the stomatal limitation of CO₂ supply and reduction of PSII photochemical efficiency, which led to the decrease in dry matter accumulation and grain yield. H₂O₂ priming increased the number of opening stomas, the stomatal length, and width, thus increasing Ci by 12.1%, which enhanced the Pn by 37.5%. Additionally, H₂O₂ priming could improve the energy of dark reaction carbohydrates by increasing the light energy absorption and utilization, alleviating the function of PSII reaction centers, protecting the PSII receptor and donor side, and the electron transport chain. The φEo, φPo, φRo, and Ψo of H-W were increased by 89.9%, 16.2%, 55.4%, and 63.9% respectively, and the φDo was decreased by 23.5%, compared with C-W. Therefore, H₂O₂ priming significantly enhanced the PSII photochemical efficiency, and increased the CO₂ supply in dark reactions to promote carbon assimilation, alleviating the waterlogging-induced damage to maize plant growth and grain yield.

[Effects of Straw Returning and Biochar Application on Summer Maize Yield and Soil N2O Emission in Guanzhong Plain]

Here, we investigated the effects of straw returning combined with biochar application on summer maize yield and soil nitrous oxide (N₂O) emissions, based on a field location trial in the Guanzhong Plain from 2019 to 2020. The soil N₂O emission rates were monitored using the static chamber-chromatography method. A comprehensive analysis of summer maize yields, soil N₂O emissions, and soil labile nitrogen components was conducted to clarify the effects of straw returning combined with biochar application on improving soil fertility, increasing summer maize yield, and reducing greenhouse gas emissions. The three treatments were no straw returning (S₀), straw returning (S), and straw returning combined with biochar application (SB). The results showed that the peak of N₂O emissions from each treatment occurred 10 d after the straw return, and the rate of soil N₂O emissions remained at a low level after 30 d of straw return. The rate of soil N₂O emissions showed highly significant positive correlations (P<0.05) with ammonium nitrogen (NH₄⁺-N), inorganic nitrogen (SIN), microbial nitrogen (MBN), and dissolved organic nitrogen (DON) contents. S significantly increased summer maize yield, cumulative N₂O emissions, yield-scaled N₂O intensity, and total nitrogen (TN) content by 7.4%-13%, 65.8%-132.2%, 54.6%-103%, and 27.8%-33%, respectively, compared to those in S₀. Although the trend for SB to increase summer maize yield (2.5%-3.3%) compared to that in S was not significant (P>0.05), SB significantly reduced cumulative N₂O emissions and yield-scaled N₂O intensity by 24.0%-27.3% and 26.4%-29.2%, respectively, compared to that in S. SB significantly reduced the rate of soil N₂O emissions by 45.1%-69.6% at the peak of N₂O emissions compared to that in S. Biochar application mitigated soil N₂O emissions induced by straw return and had a peak-shaving effect. SB significantly increased soil total N by 9.1%-12.2% compared to that in S. Combining summer maize yield, soil N₂O emissions, and TN content, SB not only improved soil fertility and summer maize yield but also reduced yield-scaled N₂O intensity, making it a suitable management practice that can be replicated to balance crop yield and environmental friendliness.

Adjusting sowing date improves the photosynthetic capacity and grain yield by optimizing temperature condition around flowering of summer maize in the North China Plain

Adjusting the sowing date to optimize temperature conditions is a helpful strategy for mitigating the adverse impact of high temperature on summer maize growth in the North China Plain (NCP). However, the physiological processes of variation in summer maize yield with sowing date-associated changes in temperature conditions around flowering remain to be poorly understood. In this study, field experiments with two maize varieties and three sowing dates (early sowing date, SD1, 21 May; conventional sowing date, SD2, 10 June; delay sowing date, SD3, 30 June) were conducted at Xinxiang of Henan Province in 2019 and 2020. Early sowing markedly decreased the daily mean temperature (T _mean), maximum temperature (T _max), and minimum temperature (T _min) during pre-silking, while delay sowing markedly decreased those temperatures during post-silking. Under these temperature conditions, both varieties under SD1 at 12-leaf stage (V12) and silking stage (R1) while under SD3 at R1 and milking stage (R3) possessed significantly lower malondialdehyde (MDA) content in leaf due to higher activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) compared to SD2. Therefore, SD1 at V12 and R1 stages and SD3 at R1 and R3 stages for both varieties showed significantly higher photosynthetic capacity, including higher SPAD, F _v /F _m, P _n, T _r, and G _s, which promoted greater pre-silking dry matter (DM) accumulation for SD1 to increase the kernel number, and promoted greater post-silking DM accumulation for SD3 to increase the kernel weight, eventually increased the grain yield of SD1 and SD3 compared to SD2. Results of regression analysis demonstrated that T _mean, T _max, and T _min values from V12 to R1 stages lower than 26.6, 32.5, and 20.3°C are necessary for improving the kernel number, while T _mean, T _max, T _min, and accumulated temperature (AT) values from R1 to R3 stages lower than 23.2, 28.9, 17.3, and 288.6°C are necessary for improving the kernel weight. Overall, optimal temperature conditions around flowering can be obtained by early (21 May) or delay (30 June) sowing to improve the kernel number or kernel weight due to improved photosynthetic capacity, eventually increasing the grain yield of summer maize in the NCP.

Nitrapyrin Mitigates Nitrous Oxide Emissions, and Improves Maize Yield and Nitrogen Efficiency under Waterlogged Field

In order to explore the effects of nitrapyrin (N-Serve) application on greenhouse gas emission and nitrogen (N) leaching of a waterlogged maize (Zea mays L.) field, we investigated the effects of applying nitrapyrin on soil ammonium (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) content, nitrous oxide (N₂O) fluxes, and the warming potential (GWP_N2O) in a waterlogged maize field. The design included three treatments: waterlogging treatment with only urea application (V-3WL), waterlogging treatment with urea and nitrapyrin application (V-3WL+N), and no waterlogging treatment applying only urea (CK). Our results revealed that waterlogging led to the increase of nitrate concentrations across the soil profile, thus potentially increasing N leaching and decreasing N use efficiency. The accumulated N₂O emissions increased significantly in waterlogged plots compared to control plots, and maximum N₂O emission fluxes occurred during the process of soil drying after waterlogging; this resulted in an increase in GWP_N2O and N₂O greenhouse gas intensity (GHGI_N2O) by 299% and 504%, respectively, compared to those of CK. However, nitrapyrin application was able to reduce N₂O emissions. Nitrapyrin application was also good for decreasing GWP_N2O and GHGI_N2O by 34% and 50%, respectively, compared to V-3WL. In addition, nitrapyrin application was conducive to reduce N leaching and improve N use efficiency, resulting in a yield increase by 34%, compared to that of V-3WL. The application of nitrapyrin helped to mitigate agriculture-source greenhouse effects and N leaching induced by waterlogging, and was a high N-efficient fertilizer method for a waterlogged field.

Different Responses in Root Water Uptake of Summer Maize to Planting Density and Nitrogen Fertilization

Modifying farming practices combined with breeding has the potential to improve water and nutrient use efficiency by regulating root growth, but achieving this goal requires phenotyping the roots, including their architecture and ability to take up water and nutrients from different soil layers. This is challenging due to the difficulty of in situ root measurement and opaqueness of the soil. Using stable isotopes and soil coring, we calculated the change in root water uptake of summer maize in response to planting density and nitrogen fertilization in a 2-year field experiment. We periodically measured root-length density, soil moisture content, and stable isotopes δ¹⁸O and δD in the plant stem, soil water, and precipitation concurrently and calculated the root water uptake based on the mass balance of the isotopes and the Bayesian inference method coupled with the Markov Chain Monte Carlo simulation. The results show that the root water uptake increased asymptotically with root-length density and that nitrogen application affected the locations in soil from which the roots acquired water more significantly than planting density. In particular, we find that reducing nitrogen application promoted root penetration to access subsoil nutrients and consequently enhanced their water uptake from the subsoil, while increasing planting density benefited water uptake of the roots in the topsoil. These findings reveal that it is possible to manipulate plant density and fertilization to improve water and nutrient use efficiency of the summer maize and the results thus have imperative implications for agricultural production.

Exogenous 6-Benzyladenine Improved the Ear Differentiation of Waterlogged Summer Maize by Regulating the Metabolism of Hormone and Sugar

Waterlogging (W-B) is a major abiotic stress during the growth cycle of maize production in Huang-huai-hai plain of China, threatening food security. A wide range of studies suggests that the application of 6-benzyladenine (6-BA) can mitigate the W-B effects on crops. However, the mechanisms underlying this process remain unclear. In this study, the application of 6-BA that effectively increased the yield of summer maize was confirmed to be related to the hormone and sugar metabolism. At the florets differentiation stage, application of 6-BA increased the content of trans-zeatin (TZ, + 59.3%) and salicylic acid (SA, + 285.5%) of ears to induce the activity of invertase, thus establishing sink strength. During the phase of sexual organ formation, the TZ content of ear leaves, spike nodes, and ears was increased by 24.2, 64.2, and 46.1%, respectively, in W-B treatment, compared with that of W. Accordingly, the sugar metabolism of summer maize was also improved. Therefore, the structure of the spike node was improved, promoting the translocation of carbon assimilations toward the ears and the development of ears and filaments. Thus the number of fertilized florets, grain number, and yield were increased by the application of 6-BA.

6-Benzyladenine increasing subsequent waterlogging-induced waterlogging tolerance of summer maize by increasing hormone signal transduction

Summer maize is frequently subjected to waterlogging damage because of increased and variable rainfall during the growing season. The application of 6-benzyladenine (6-BA) can effectively mitigate the waterlogging effects on plant growth and increase the grain yield of waterlogged summer maize. However, the mechanisms underlying this process and the involvement of 6-BA in relevant signal transduction pathways remain unclear. In this study, we explored the effects of 6-BA on waterlogged summer maize using a phosphoproteomic technique to better understand the mechanism by which summer maize growth improves following waterlogging. Application of 6-BA inhibited the waterlogging-induced increase in abscisic acid (ABA) content and increased the phosphorylation levels of proteins involved in ABA signaling; accordingly, stomatal responsiveness to exogenous ABA increased. In addition, the application of 6-BA had a long-term effect on signal transduction pathways and contributed to rapid responses to subsequent stresses. Plants primed with 6-BA accumulated more ethylene and jasmonic acid in response to subsequent waterlogging; accordingly, leaf SPAD, antioxidase activity, and root traits improved by 6-BA priming. These results suggest that the effects of 6-BA on hormone signal transduction pathways are anamnestic, which enables plants to show faster or stronger defense responses to stress.

Effects of basic application of chlorocholine chloride combined with nitrogen fertilizer on nitrogen use of summer maize in North China Plain

To clarify the effects of combined applications of chlorocholine chloride (CCC) and nitrogen fertilizer (CN) on nitrogen metabolism and nitrogen use efficiency of summer maize, we conducted a field experiment in Xinxiang experimental station of Chinese Academy of Agricultural Sciences in 2018 and 2019, with four nitrogen application rates (0, 62.5, 125 and 187.5 kg·hm^-2), and two maize varieties of Jingnongke 728 (JNK728) and Zhongdan 909 (ZD909). The results showed that across the two years CN-CCC increased maize yield by 7.7% and 5.0% under the nitrogen application rates of 62.5 kg·hm^-2 and 125 kg·hm^-2, respectively. CN-CCC increased the contents of nitrate reductase, glutamine synthetase, glutamate synthetase and soluble protein, and finally promoted nitrogen metabolism. Under the low and middle nitrogen application conditions (62.5 kg·hm^-2 and 125 kg·hm^-2), plant nitrogen content of JNK728 and ZD909 increased by 17.6% and 30.3%, grain nitrogen content increased by 10.3% and 17.4%, nitrogen partial productivity, agronomic efficiency of applied nitrogen, recovery efficiency of applied nitrogen, nitrogen use efficiency increased by 10.0%, 15.7%, 23.3%, 24.8% and 5.7%, 15.0%, 49.9%, 71.7%, respectively. In conclusion, appropriate basic application of CN-CCC could enhance nitrogen metabolism, increase nitrogen use efficiency and grain yield of summer maize. Our results showed that CCC combined basic nitrogen application of 125 kg·hm^-2 had the best effect.

[Effects of Straw Mulching and Nitrogen Reduction on the Distribution of Soil Nitrogen and Groundwater Nitrogen Pollution]

To explore the effects of straw mulching and reduced nitrogen fertilization on the temporal and spatial patterns of soil nitrogen, groundwater nitrogen pollution, and summer maize yield, field experiments were carried out in the Hetao irrigation district in 2017 and 2018. The experiment involved the following seven treatments:a control (CK) treatment involving conventional fertilization and traditional tillage, and conventional nitrogen applications reduced by 30% (N1), 20% (N2), and 10% (N3) coupled with either straw surface covering (B) or deep straw burial (S). The results showed that the distribution of soil nitrogen in the CK treatment varied depending on soil depth, with an overall decreasing trend. In the 0-20 cm soil layer under straw surface covering (B) treatments, soil nitrogen was superficially accumulated. NO₃^--N and NH₄⁺-N content increased by an average of 22.2% and 42.7% compared to the CK treatment, respectively, which decreased significantly at first and then increased slightly with depth. In the 20-40 cm deep soil layer under the deep straw burial (S) treatments, soil nitrogen accumulated and the content of NO₃^--N and NH₄⁺-N increased by an average of 29.8% and 48.1%, respectively, compared to the CK treatment. Nitrogen accumulation first and then decreased significantly with depth. Nitrogen accumulation under the different straw mulching regimes increased with an increase in the application of reduced nitrogen. After the harvest of summer maize, the accumulation of NO₃^--N and NH₄⁺-N in the >80 cm soil layer under the B treatments was 19.9%-58.2% and 31.1%-61.7% lower than that of the CK treatment, respectively. This compared to reductions of 36.7%-70.9% and 82.6%-89.2% for the S treatments, respectively. Only the BN3 treatment increased accumulation compared with CK by 0.4% on average, while the SN2 treatment resulted in a 9.3% increase. Summer maize yield and relative indexes were also improved relative to the other treatments. Nonlinear fitting of yield and application reduction showed that deep straw burial was better than surface covering at increasing summer maize production. The effect of deep straw burial and 14%-20% application reduction was better. Straw mulching with reduced nitrogen fertilization can limit nitrogen leaching and thereby reduce the risk of groundwater pollution. After the harvest, groundwater quality was classified in the Ⅱ class, with the risk of nitrogen contamination being lowest under deep straw burial with>20% reduced nitrogen fertilization. These observations show that deep straw deep alongside 14%-20% application reduction could effectively alleviate nitrogen leaching and reduce the risk of nitrogen pollution in groundwater. This approach can help improve the ecological environment and summer maize yields in the Hetao irrigation district.

[Impacts of Nitrogen Application on Ammonia Volatilization During Maize Season in Northern China]

Ammonia volatilization is one of the major paths of nitrogen (N) loss and may exert a substantial impact on air quality. This study aims to explore the effects of nitrogen (N) fertilizer types, fertilization rate, and application timing and gas collection method on NH₃ volatilization during the maize season in Northern China. This study collected the publications on the NH₃ volatilization from maize farming which were conducted in Northern China from 1980 to 2018, and undertook a systematic analysis. The study found that with the increase of N rate, the total and net NH₃ volatilization at the basal and topdressing fertilization stages increased at exponential and power function, respectively. When the ratio of basal/topdressing N rate was 1/1, the total and net NH₃ volatilization during the topdressing stage (58.4% of the whole season emission) was significantly higher than that in the basal fertilization stage (41.6%) (P<0.05). The priming effect first showed a negative effect and then gradually turned into a positive effect with the increase of N rate. Due to the positive priming effect, the net NH₃ volatilization, without considering the priming effect, was overestimated under the conventional N application (>297 kg·hm^-2). There is a significant difference between the NH₃ volatilization measured by the venting method and the sponge absorption method, and the data from the venting method are more stable (P<0.01). Compared with conventional urea, slow-release urea may reduce NH₃ volatilization by 20% to 50%. Control fertilizer N rate at the topdressing stage is more efficient in reducing the NH₃ volatilization from maize production in Northern China, and the venting method is more suitable for the quantification of NH₃ volatilization than the sponge absorption method under a high rate of fertilizer N.

[Effects of phytase Q9 on the yield and senescence characteristics of summer maize shaded in the field]

Light shortage in the canopy of summer maize resulted from the decrease of solar radiation and the increase of planting density in Huanghuaihai region could reduce maize yield. In order to explore the effects of phytase Q9 on leaf senescence characteristics of summer maize, three sha-ding treatments with summer maize hybrid 'Denghai 605' (DH605) were conducted, including shading at flowering to maturity stage (S₁), shading at ear stage (S₂), and shading at whole growth stage (S₃) with natural lighting in the field as control (CK). Chemical control reagent phytase Q9 was used to regulate the shading treatments (the original solution was diluted by 100 times) and the CK exogenously, namely shading at flowering to maturity stage-phytase Q9 (S₁Q), shading at ear stage-phytase Q9 (S₂Q), and shading at whole growth stage-phytase Q9 (S₃Q), and natural lighting-phytase Q9 (CKQ), with spraying water at the same stage as the control. The results showed that leaf area index (LAI), soil-plant analysis development (SPAD) value and net photosynthetic rate of summer maize was significantly reduced by shading, which led to decreases of yield. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) in ear-leaf decreased. The content of malondialdehyde (MDA) and free proline increased, and that of soluble protein decreased. The spraying phytase Q9 significantly increased LAI, SPAD and net photosynthetic rate in S₃Q and S₂Q. MDA and free proline content in S₃Q, S₂Q, and S₁Q were significantly decreased, and soluble protein content and POD activity was significantly increased. SOD and CAT activities in S₂Q and S₃Q were significantly increased. The yield of S₃Q, S₂Q and S₁Q were 19%, 8% and 7% higher than that of S₃, S₂ and S₁ respectively. There was no significant difference between CKQ and CK. In conclusion, phytase Q9 could effectively alleviate the negative impact of low light on yield formation of summer maize, and increase grain yield by improving photosynthetic capacity of leaves and delaying leaf senescence.

[Effects of spraying desiccant on dehydration characteristics and grain quality of summer maize hybrids differing in maturity]

Water content of summer maize hybrids grown in China is too high at harvesting stage, which limits the development of grain mechanical harvesting technology. Spraying the desiccant can regulate physiological process of crop grain filling and reduce water content at harvest. We explored the effects of spraying the desiccant on the dehydration process, grain moisture, and grain quality of summer maize hybrids differing in maturity. Spraying the desiccants reduced dry matter accumulation in different organs of maize, with strongest reduction of middle-late maturity hybrids. Dry matter transfer to the grains of the plants and the harvest index was improved, but with no changes of grain quality. The dehydration rate of grains was positively correlated with the rate of dehydration in diffe-rent organs. The dehydration rate of grains after spraying the desiccants was significantly positively correlated with the rate of dehydration of stems and sheaths. With no negative effects on yield, spraying the desiccant increased the total dehydration rate, shortened the time from flowering to physiological maturity, and increased the time from physiological maturity to harvest, which was beneficial to the further reduction of grain moisture in the later stage. The possibility of grain mechanical harvesting was increased. The economic benefits of spraying the desiccants on mechanical grain harvest of summer maize hybrids differing in maturity were not significantly different from those of ear mechanical harvesting. The economic benefits of middle-late maturity hybrids were higher than those of early maturity hybrids. Spraying desiccant may improve the possibility of grain mechanical harvesting.

[Effects of nitrogen application rate on grain filling characteristics and nutritional quality of summer maize.]

Suitable nitrogen application rate can significantly increase grain filling rate and yield and improve nutritional quality. Denghai 518 (DH518) and Zhengdan 958 (ZD958) were used as experimental materials in this study. A field experiment with four treatments, no nitrogen treatment (N₀), decrement nitrogen application rate (N₁, 129 kg N·hm^-2), suitable nitrogen rate (N₂, 184.5 kg N·hm^-2) and excessive nitrogen rate (N₃, 300 kg N·hm^-2), was conducted to explore the effects of nitrogen fertilization on grain filling parameters and nutritional qualities of summer maize. Results showed that grain filling characteristics, grain dry weight, and yield in N₀ treatment was decreased. With increasing nitrogen application rate in the suitable range, average filling rate, grain dry weight, and yield increased. Grain yield of two hybrids in N₁ and N₂ treatments was higher than that of N₀ by 16.4%-57.2% and 35.8%-65.1%, respectively. Grain protein, soluble sugar, starch contents and the ratio of amylopectin and amylase contents were lower and crude fat content was higher in N₀ treatment. Grain protein content, soluble sugar content and starch content in N₂ of DH518 were higher than that of N₀ and N₁ by 32.5% and 6.5%, 19.9% and 9.5%, 8.9% and 5.2%, and the ratio of amylopectin and amylose contents was increased. Grain protein, soluble sugar and starch contents in N₂ of ZD958 were higher than that of N₀ and N₁ by 16.9% and 7.8%, 30.5% and 14.8%, 11.5% and 5.7%, and the ratio of amylopectin and amylase contents was increased. Crude fat content in N₂ of both hybrids decreased significantly by 4.8%-12.3% than that of N₀ and N₁. However, yield and nutritional quality was increased in N₃ treatment than that of N₂. Our results suggested that suitable nitrogen rate could enhance grain filling, grain weight, and grain nutritional quality.

Dynamic Zinc Accumulation and Contributions of Pre- and/or Post-Silking Zinc Uptake to Grain Zinc of Maize as Affected by Nitrogen Supply

Nitrogen (N) supply could improve the grain yield of maize, which is of great importance to provide calories and nutrients in the diets of both humans and animals. Field experiments were conducted in 2009 and 2010 to investigate dynamic zinc (Zn) accumulation and the pre-silking and post-silking Zn uptake and their contributions to grain Zn accumulation of maize with different N supply under field conditions. Results showed that only 1.2% to 39.4% of grain Zn accumulation derived from pre-silking Zn uptake, with Zn remobilization being negatively affected by increasing N supply. However, post-silking Zn uptake (0.8-2.3 mg plant-1) and its substantial contribution to grain Zn accumulation (60.6%-98.8%) were progressively enhanced with the increasing N supply. Furthermore, grain Zn concentration was positively associated with grain N concentration (r = 0.752***), post-silking N uptake (r = 0.695***), and post-silking Zn uptake (r = 738***). A significant positive relationship was also found between post-silking uptake of N and Zn (r = 0.775***). These results suggest that N nutrition is a critical factor for shoot Zn uptake and its allocation to maize grain. Dry weight, and N and Zn concentration of grain and straw were significantly enhanced with the increasing N from "no N" to "optimal N" supply (150 kg N ha-1 in 2009 and 105 kg N ha-1 in 2010), but further increasing N supply (250 kg N ha-1) generally resulted in a non-significant increase in both cropping seasons. During the grain development, N supply also generally tended to improve grain N and Zn concentrations, but decrease phosphorus (P) concentration and the molar ratio of P to Zn compared with null N application. These results suggest that grain Zn accumulation mainly originates from post-silking Zn uptake. Applying N at optimal rates ensures better shoot Zn nutrition and contributes to post-silking Zn uptake, maintaining higher grain Zn availability by decreasing the molar ratio of P to Zn, and resulting in benefits to human nutrition.

[Effects of directional planting on light environment and leaf photosynthesis of summer maize population]

To improve light environment, photosynthetic capacity, and thus the yield of maize, the effects of directional planting on light distribution in canopy and photosynthetic characteristics of ear leaves, as well as the performance of PSII that closely related with photosynthetic characteristics and reflected by the rapid chlorophyll fluorescence kinetic curves were examined in Zhengdan 958 maize variety. The results showed that the orientation of leaves remarkably affected photosynthetically active radiation (PAR) interception of ear leaves, with PAR interception of ear leaves in southward treatment being 271.8% higher than that under northward treatment. The orientation of leaves affec-ted photosynthetic light use efficiency of ear leaves under high and low light conditions. The southward treatment increased net photosynthetic rate (P_n) under saturated light in ear leaves, indicating that the use efficiency to high light was enhanced in leaves of southward treatment. In contrast, the northward treatment increased the apparent quantum yield (α) of ear leaves, indicating leaves in southward treatment adapted the light-limited environment. During the early stage after anthesis, the performance of PSII electron donor side and electron acceptor side was significantly improved, and thus enhanced the performance of PSII reaction center (PI_ABS) and fluorescence photochemical quenching coefficient (Ψ_o) in ear leaves of southward treatment. The increase of quantum yield of electron transfer (φ_Eo) indicated the enhancement of transfer performance of electrons from photosystem 2 (PSII) to photosystem 1 (PSI) in leaves of southward treatment. The photosynthetic performance of ear leaves showed a trend of southward > eastward > westward > northward during the early stage after anthesis. Forty days after anthesis, the use efficiency to high light decreased in ear leaves of southward treatment, but the ear leaves of southward treatment showed high use efficiency to low light, which changed the trend of photosynthetic performance of ear leaves to northward > westward > eastward > southward. In summary, northward and eastward treatments improved the light distribution in canopy, the PAR interception of ear leaves, the capacity of photosynthesis and dry matter production, and consequently increased the yield of summer maize.

[Fraction of absorbed photosynthetically active radiation over summer maize canopy estimated by hyperspectral remote sensing under different drought conditions.]

Fraction of absorbed photosynthetically active radiation (fAPAR) is one of the important remote sensing model parameters of vegetation productivity. However, the crop canopy fAPAR estimation during growing season under different drought conditions has not been reported yet. In this study, the characteristics of summer maize canopy fAPAR and spectral reflectance during growing season under different drought stresses and the relationships of fAPAR with reflectance, the first derivative spectral reflectance and vegetation indices were examined based on the hyperspectral reflectance and fAPAR data from the summer maize drought manipulation experiment with five irrigation levels in 2015. Under mild water stress and sufficient water supply conditions, fAPAR was higher, with the maximum value of 0.7. Under severe water stress and severe persistent drought, fAPAR was lower, with the minimum value of 0.06. Reflectance of visible and shortwave bands increased and near infrared reflectance decreased with increasing drought. The fAPAR was negatively related with visible bands and shortwave bands, but positively correlated with near infrared. Visible and shortwave band reflectance had significant correlation with fAPAR, especially at 383, 680 and 1980 nm, with all the correlation coefficients being more than -0.87. The strong and stable relationship between the first derivative spectral reflectance and fAPAR appeared at 580, 720 and 1546 nm, with the correlation coefficients being -0.91, 0.89 and 0.88, respectively. There were linear or logarithm relationships between fAPAR with nine vegetation indices. Among the nine indices, the enhanced vegetation index (EVI), renormalized difference vegetation index (RDVI), soil adjusted vegetation index (SAVI), and modified soil adjusted vegetation index (MSAVI) performed well with the correlation coefficient being higher than 0.88, and the average relative error (RMAE) 16.6%, 16.6%, 16.7% and 16.2%, respectively. Based on the logarithmic relationship between first derivative spectral reflectance and fAPAR, the simulation effect was best at the band of (720±5) nm, with a correlation coefficient of 0.86. The correlation coefficient of the relationship between fAPAR and reflectance was less than 0.81. The results could provide fAPAR simulation for remote sensing model of vegetation productivity and drought warning.

[Influencing factors and their simulation of summer maize land surface-air temperature difference under drought conditions]

Crop water deficit status characterized by land surface-air temperature difference (T_s-T_a) has been widely investigated. However, empirical evidence for characteristics and impact factors of T_s-T_a considering the process of crop growth are less yet, which restricts the accurate simulation of T_s-T_a. Here, the data of T_s-T_a during the process of maize growth were obtained from five irrigation water control experiments after the period of summer maize 3-leaf stage in 2014 and jointing stage in 2015. The results showed that T_s-T_a of summer maize cropland was significantly affected by soil water content. T_s-T_a increased with the deficit of soil water. During summer maize water treatments, the normalized difference vegetation index (NDVI) was the main impact factor of T_s-T_a, with a significant linear relationship. However, during different growth stages, some additional factors including meteorological, biological and soil factors could also affect T_s-T_a, including canopy photosynthetic active radiation absorption ratio (f_APAR) after 3-leaf stage, relative soil water content (RSWC), and air relative humidity (RH) from 3-leaf stage to jointing stage. Then, the growth duration simulation model of T_s-T_a, vegetative growth simulation model of T_s-T_a and reproductive growth simulation model of T_s-T_a were established in terms of the data in 2014. Those simulation models were validated based on the experimental data of five irrigation water treatments after summer maize jointing stage in 2015. The results showed that the growth duration simulation mode of T_s-T_a could explain 63% variation of T_s-T_a in 2015. However, 79% variation of T_s-T_a could be explained by the simulation results of the vegetative growth simulation model of T_s-T_a and the reproductive growth simulation model of T_s-T_a. The results provided the basis for the quantitative evaluation of crop drought based on T_s-T_a.

[Response of dry matter partitioning coefficient of summer maize to drought stress in North China]

The dry matter partitioning coefficient (PC) reflects the distribution and accumulation of dry matter in crop organs. Understanding the responses of PC to drought stress is fundamental for understanding crop development under drought stress. Using field data collected under drought stress conditions during the period 2013-2015 at three sites (Xiajin, Shandong; Gucheng, Hebei; and Yuncheng, Shanxi) in North China, we quantified the effects of different drought stress intensities on the PC of stems, leaves and ears during the development stages (from emergence to jointing, from jointing to tasseling, and from tasseling to maturity). The results showed that PC of stems, leaves and ears showed significant quadratic relationships with relative soil moisture during all stages. Leaf PC was increased by 0.04-0.09 during the grain-filling stage under light and moderate drought stress, and was increased by 0.17 during the tasseling stage under sever drought stress. On the contrary, drought stress had negative effects on the ear PC. With the increases of drought stress, less dry matter was allocated to ears. Under light to severe drought stresses, ear PC was decreased by 0.08-0.34. However, no consis-tent pattern was observed for the effects of drought stress on stem PC: negative effects were observed during the vegetative growth stage and positive effects were found in the mature stage. Overall, the degree of the responses of crop PC to drought stress under different phenology stages was in the order of grain-filling stage (positive) &gt; tasseling stage (negative) &gt; seedling stage (negative).

Relationship between fluorescence yield and photochemical yield under water stress and intermediate light conditions

The dynamics between fluorescence (Fs) yield and photochemical (P) yield in a changing environment are essential for understanding the relationship between photosynthesis and fluorescence. The ratio of Fs yield and P yield tends to be constant under high light intensity, but the relationship between these two yields, and its response to environmental conditions, need to be explored further under intermediate and low light. In this study, we performed leaf-scale measurements of fluorescence parameters by pulse-amplitude modulation (PAM) technology in summer maize (Zea mays L.) plants grown under intermediate light conditions in a climate chamber. Plants were treated as moderately water stressed and non-water stressed. Results showed that a decrease in P yield was accompanied by increases in Fs yield and non-photochemical quenching (NPQ) yield in response to moderate water stress under intermediate and low light conditions. Fs yield was negatively correlated with P yield under intermediate and low light conditions when there was sufficient soil water in the root zone. Under water stress, the correlation between Fs yield and P yield was negative in low light, but became positive under higher light levels. Fs yield was negatively related to P yield when NPQ yield was low; however, they were synergistically and positively associated when excessive light dissipation was dominated by NPQ.

[Effects of Plastic Film Mulching Patterns and Irrigation on Yield of Summer Maize and Greenhouse Gas Emissions Intensity of Field]

In order to evaluate the effect of different treatments on yield and greenhouse gas emissions during the summer maize growing season, a two-year film mulching experiment was conducted in 2014 and 2015. In this experiment, the two main experimental factors were rainfed treatment (R) and irrigated treatment (I), and the secondary experimental factors included control treatment (CK), half film mulching treatment (HM), and full film mulching treatment (FM). The emissions of soil greenhouse gases (CO₂, CH₄, and N₂O) were monitored using a static opaque chamber and chromatography method. Moreover, the greenhouse gas emissions intensity (GHGI) was used to evaluate the effect of carbon sequestration in different treatments. The results of this study showed that the yields of the RHM and RFM treatments did not differ significantly in 2014, but increased by 19.6% and 26.8%, respectively, in 2015 compared with that of RCK. The yield of IHM was not improved, and that of IFM significantly increased by 14.1% and 55.8% in 2014 and 2015, respectively, compared with that of ICK. The irrigated treatments only promoted CO₂ emissions in 2015 (P<0.01), and all film mulching treatments (regardless of HM and FM treatments) had no effect on CO₂ emissions under rainfed and irrigated conditions (P>0.05). Irrigated treatments had no effect on the absorption of CH₄ (P>0.05), whereas the film mulching treatments had an inhibitory effect. Compared with values of RCK, the amount of seasonal N₂O emissions for ICK showed a significant difference in 2015 with a decrease of 22.3%. Compared with values of RCK, the amounts of N₂O emissions for RHM and RFM had no significant differences in 2014, but significantly decreased by 50.7% and 51.4% in 2015, respectively. Compared with ICK, IHM and IFM significantly decreased the amounts of N₂O emissions by 47.5% and 54.2% in 2014, and by 9.6% and 52.2% in 2015, respectively. The GHGIs of RHM and RFM were significantly reduced by 60.1% and 61.7% in 2015, respectively, compared with values of RCK, and the GHGIs of IHM and IFM were significantly reduced by 39.7% and 53.2% in 2014, and reduced by 22.2% and 67.5% in 2015, respectively, compared with that of ICK. This means that the effect of FM on reducing GHGI was better than that of HM. It was also found that the significantly reduced GHGI in irrigated treatments may be attributed to the increased yields. Therefore, FM under irrigation conditions was recommended for summer maize for stabilizing the yield and reducing the GHGI.

Effects of different densities of mixed-cropping on 13C-photosynthate distribution and grain yield of maize

To explore the effects of different densities of mixed-cropping on ¹³C-photosynthate distribution and grain yield of maize, we measured photosynthetic characteristics, ¹³C-photosynthate distribution, dry matter accumulation, and grain yield under different planting densities (LD, 67500 plants·hm^-2 and HD, 97500 plants·hm^-2) under mixed-cropping (M, 1:1, 2:2) and monoculture of Zhengdan958 (ZD) and Denghai605 (DH). The results showed that with the increases of planting density, grain yield, ¹³C-photosynthate allocation to grain, dry matter accumulation, and leaf area index (LAI) increased, but the chlorophyll content and net photosynthetic rate decreased. No significant difference was observed between the monoculture and mixed-cropping at the density of 67500 plants·hm^-2. However, at 97500 plants·hm^-2, LAI, chlorophyll content, net photosynthetic rate and dry matter accumulation in mixed-cropping were higher than that in monoculture. Mixed-cropping promoted the transport of dry matter from the vegetative organs such as stem to the grain and the distribution of ¹³C-photosynthates to grain. Grain yield of summer maize significantly increased in mixed-cropping due to the increase of 1000-grain mass. Under high plant density, the mixed-cropping could enlarge photosynthetic area, maintain higher net photosynthetic rate, increase dry matter accumulation, improve the distribution of dry matter, promote the distribution of ¹³C-photosynthates to grains and thus increase the grain yield. Our results indicated that mixed-cropping could significantly increase the yield of close planting summer maize in Huang-Huai-Hai Plain.

)]

We examined the changes of photosynthetic characteristics and chloroplast ultrastructure in mesophyll cell of summer maize in response to different light intensities in the field, with the summer maize hybrid Denghai 605 as experimental material. Two treatments of both shading (S) and increasing light (L) from flowering to physiological maturity stage were designed, with the ambient sunlight treatment as control (CK). Under shading treatment, poorly developed thylakoid structure, blurry lamellar structure, loose granum, large gap between slices and warping granum were the major characteristics in chloroplast. Meanwhile, photosynthetic rate (P_n), transpiration rate, stomatal conductance, chlorophyll content, and actual photo-chemical efficiency (Φ_PSII) decreased, whereas the maximal photochemical efficiency and non-photochemical quenching increased, which resulted in decreases in grain yield under shading treatment. However, a better development was observed in chloroplasts for L treatment, with the number of grana and lamellae increased and lamellae arranged compactly. In addition, P_n and Φ_PSII increased under L treatment, which increased grain yield. The chloroplast arrangement dispersed in mesophyll cells and chloroplast ultrastructure was destroyed after shading, and then chlorophyll synthesis per unit leaf area and photosynthetic capacity decreased. In contrast, the number of grana and lamellae increased and lamellae arranged compactly after increasing light, which are beneficial for corn yield.

Large reductions in pesticides made possible by use of an insect-trapping lamp: a case study in a winter wheat-summer maize rotation system

BACKGROUND

Increasing attention is being paid to physical methods to control pests such as insect trapping. In order to examine how pesticides can reasonably be combined with the use of an insect-trapping lamp and by how much this can reduce the amount of pesticide used, five treatments were applied to a winter wheat-summer maize rotation system in eastern China: a treatment in which only pesticides were used; a treatment with only insect-trapping lamps; insect-trapping lamps plus one application of pesticides; insect-trapping lamps plus two applications of pesticides; insect-trapping lamps plus three applications of pesticides.

RESULTS

The results showed that, when pesticides were reduced by 25-35%, the insect-trapping lamps controlled the insect population well and yields were not decreased but were actually increased, with pesticides being applied only at 2 days before winter wheat planting, at winter wheat flowering and at the big flare stage of summer maize. Reducing pesticides by 35-65% had no adverse effect on crop yields, and thus had the potential to reduce the costs of pest control and produce the greatest economic benefit. When no pesticides were used in the insect-trapping lamp control area, the annual yield was still >15 t hm^-2 .

CONCLUSION

If pesticides are used in a timely fashion and at the appropriate stage, their use may be greatly reduced with the help of an insect-trapping lamp. © 2018 Society of Chemical Industry.

Photosynthetic Characteristics and Chloroplast Ultrastructure of Summer Maize Response to Different Nitrogen Supplies

Maize (Zea mays L.) is the important crop over the world. Nitrogen (N) as necessary element affects photosynthetic characteristics and grain yield of summer maize. In this study, N0 (0 kg N ha^-1), N1 (129 kg N ha^-1), N2 (185 kg N ha^-1), and N3 (300 kg N ha^-1) was conducted using hybrid 'ZhengDan958' at Dawenkou research field (36°11'N, 117°06'E, 178 m altitude) in the North China Plain to explore the effects of N rate on photosynthetic characteristics and chloroplast ultrastructure. Gas exchange parameters, chlorophyll fluorescence parameters, leaf area index (LAI), chlorophyll SPAD value, chloroplast ultrastructure, dry matter weight and grain yield were measured. At physiological maturity stage, dry matter weight and grain yield of N2 increased by 33-52% (P ≤ 0.05) and 6-32% (P ≤ 0.05), respectively, compared with other treatments. During the growing from silking (R1) to milk (R3) stage, LAI of N0 and N1 were 35-38% (P ≤ 0.05) and 9-23% (P ≤ 0.05) less than that of N2, respectively. Chlorophyll SPAD value of N0 and N1 were 13-22% (P ≤ 0.05) and 5-11% (P ≤ 0.05) lower than that of N2. There was no significant difference in LAI and chlorophyll SPAD value between N2 and N3 during the period from R1 to R3 (P > 0.05). The net photosynthetic rate (P_n), maximal quantum efficiency of PSII (F_v/F_m) and quantum efficiency of PSII (Φ_PSII) were higher with the increase of N rate up to N2 (P ≤ 0.05), and those of N3 were significantly less than N2 (P ≤ 0.05). In compared with N2, the chloroplast configuration of N0 and N1 became elliptical, almost circular or irregular. The membrane of chloroplast and thylakoid resolved with growing stage, and the number of chloroplast per cell and lamellae per grana decreased under N0 and N1 treatment (P ≤ 0.05). Under N0 and N1 treatments, summer maize had more negative photosynthetic characteristics. The more number of osmium granule and vesicle and the larger gap between lamellae were shown in N3. Therefore, N2 treatment, 185 kg N ha^-1, is the appropriate application rate for grain yield, photosynthesis and chloroplast ultrastructure.

Simple Assessment of Nitrogen Nutrition Index in Summer Maize by Using Chlorophyll Meter Readings

Rapid and non-destructive diagnostic tools to accurately assess crop nitrogen nutrition index (NNI) are imperative for improving crop nitrogen (N) diagnosis and sustaining crop production. This study was aimed to develop the relationships among NNI, leaf N gradient, chlorophyll meter (CM) readings gradient, and positional differences chlorophyll meter index [PDCMI, the ratio of CM readings between different leaf layers (LLs) of crop canopy] and to validate the accuracy and stability of these relationships across the different LLs, years, sites, and cultivars. Six multi-N rates (0-320 kg ha^-1) field experiments were conducted with four summer maize cultivars (Zhengdan958, Denghai605, Xundan20, and Denghai661) at two different sites located in China. Six summer maize plants per plot were harvested at each sampling stage to assess NNI, leaf N concentration and CM readings of different LLs during the vegetative growth period. The results showed that the leaf N gradient, CM readings gradient and PDCMI of different LLs decreased, while the NNI values increased with increasing N supply. The leaf N gradient and CM readings gradient increased gradually from top to bottom of the canopy and CM readings of the bottom LL were more sensitive to changes in plant N concentration. The significantly positive relationship between NNI and CM readings of different LLs (LL1 to LL3) was observed, yet these relationships varied across the years. In contrast, the relationships between NNI and PDCMI of different LLs (LL1 to LL3) were significantly negative. The strongest relationship between PDCMI and NNI which was stable across the cultivars and years was observed for PDCMI1-3 (NNI = -5.74 × PDCMI1-3+1.5, R² = 0.76^**). Additionally, the models developed in this study were validated with the data acquired from two independent experiments to assess their accuracy of prediction. The root mean square error value of 0.1 indicated that the most accurate and robust relationship was observed between PDCMI1-3 and NNI. The projected results would help to develop a simple, non-destructive and reliable approach to accurately assess the crop N status for precisely managing N application during the growth period of summer maize crop.

[Effects of water conditions and controlled release urea on yield and leaf senescence physiological characteristics in summer maize.]

In an soil column experiment with Zhengdan 958 (a summer maize cultivar planted widely in China), treatments of three water levels,severe water stress W₁ which the soil moisture kept (35±5)% of the field capacity, mild water stress W₂ which was (55±5)%,normal water W₃ which was (75±5)%, and four levels of controlled release urea fertilizer (N₀, N₁ was 150 kg N·hm^-2,N₂ was 225 kg N·hm^-2 and N₃ was 300 kg N·hm^-2) were included to study the interactive effects of water and controlled release urea on yield and leaf senescence characteristics of summer maize. The results showed that the coupling of water and controlled release urea had significant effects on increasing yield, delaying the senescence and keeping the high efficiency of the functional leaves. Under the same nitrogen condition, yield, LAI, chlorophyll content and the activities of SOD, POD, CAT and soluble protein content in summer maize ear leaf were significantly increased with more water supplying, and the content of MDA decreased significantly. Under the condition of the same moisture, these indicators were also significantly increased with the increasing nitrogen application and MDA content was reduced significantly. However, these indicators (except MDA) of W₃N₃, W₃N₂ and W₂N₃ treatments were maintained at a higher level and the MDA content was lo-wer compared with other treatments despite the fact that there were no significant difference among these three treatments, which indicated that the interactive effects of water and controlled release urea had an important role in maintaining the function of ear leaf, delaying the leaf senescence, and was beneficial to the photosynthates production and obtaining higher yield of summer maize. Integrating the yield, LAI, chlorophyll content, various protective enzymes activity, MDA and soluble protein content, controlled release urea application rate of 225 kg N·hm^-2 was the best treatment as the soil moisture content was (75±5)% of field capacity. Continuous increase in the nitrogen application could not enhance the activities of protective enzymes, oppositely, it could cause the decline of protective enzymes activities and the increase of MDA content rapidly and speed up plants translation to senescence, which was not conductive to the efficient use of nitrogen. We suggested that coupling controlled release urea application rate of 300 kg N·hm^-2 with soil moisture content of (55±5)% of field capacity was optimum.

[Temporal variation of soil net nitrogen mineralization in summer maize growing period under plastic film mulched cultivation in Danjiangkou Reservoir Area, China.]

Soil net nitrogen mineralization in cropland has a great influence on both agricultural non-point source pollution and soil nitrogen loss. A field plot experiment was conducted to explore the temporal variation of soil net nitrogen mineralization under plastic film mulching ridge-furrow in Wulongchi small watershed during summer maize growing period. Results showed that the soil net ammonification, nitrification, and nitrogen mineralization were significantly greater than those of non-mulched treatment, and the differences were 6.63, 12.96 and 19.59 mg·kg^-1, respectively. During the summer maize growth period, the rate of soil net ammonification was high at seedling stage, low at heading stage, and high at maturation stage. Both the rates of soil net nitrification and nitrogen mineralization were high at jointing stage, low at heading stage, and high at maturation stage. The rate of soil net nitrogen mineralization under plastic film mulched had significant linear relation with the contents of soil total nitrogen, nitrate nitrogen, and soil water. In conclusion, the improved condition of soil water and temperature under plastic film mulched cultivation of summer maize in the growing period promoted soil net nitrogen mineralization.

Response of Summer Maize Photosynthate Accumulation and Distribution to Shading Stress Assessed by Using 13CO2 Stable Isotope Tracer in the Field

Maize is one of the most important crops globally that provides food, feed, and bioenergy. However, shading stress threatens maize production. In this study, we investigated the effects of shading on photosynthate accumulation and distribution of summer maize in the field. Zhengdan958 (ZD958) and Denghai 605 (DH605) were used as experimental materials in a field experiment running from 2013 to 2015. Shading treatments were applied over different growth stages: from the tassel stage (VT) to physiological maturity (R6) (S1), from the six-leaf stage (V6) to VT (S2), and from emergence stage (VE) to R6 (S3). The effects of shading on plant photosynthesis, photosynthate accumulation and distribution, and yield were evaluated in comparison to ambient sunlight. Shading significantly decreased the leaf area, SPAD value, net photosynthetic rate, dry matter accumulation, and grain yield. During the 3-year experimental period, grain yields of ZD958 and DH605 were reduced by 83.4%, 34.2%, 53.1% and 79.3%, 24.2%, 57.6% as compared to the CK by treatments S3, S2, and S1, respectively. ¹³CO₂ stable isotope tracing revealed that shading differentially affected the photosynthate transfer rate in different stages; photosynthates were transferred from top to bottom plant parts, in the order control > S2 > S1 > S3. We conclude that shading clearly disrupted photosynthate metabolism, and reduced the photosynthate accumulation in the grain, resulting in a yield reduction.

[Comprehensive Effects of the Application of Water and Fertilizer Amount on CO2 Emission from Soils of Summer-maize Field]

Field experiments were conducted to determine the effects of water and fertilizer amount on soil CO₂ emissions by using the method of static chamber/gas chromatography in summer maize farmland ecosystem. Three factors (three irrigation levels including 90 mm, 76.5 mm and 63 mm, four nitrogen fertilizer levels including 300 kg·hm^-2, 255 kg·hm^-2, 210 kg·hm^-2 and 0 kg·hm^-2, and four phosphate fertilizer levels including 90 kg·hm^-2, 76.5 kg·hm^-2, 63 kg·hm^-2 and 0 kg·hm^-2) were designed in the experiment. The results showed that soil CO₂ emission under different water and fertilizer conditions showed obvious seasonal fluctuation, the main and secondary peak appeared at jointing to tasseling stage and tasseling to grouting stage. The soil CO₂ emissions were relevant to the supply levels of fertilizer and irrigation. In the high fertilizer F₁ (N 300 kg·hm^-2, P₂O₅ 90 kg·hm^-2) and low fertilizer F_0.7 (N 210 kg·hm^-2, P₂O₅ 63 kg·hm^-2) conditions, average soil CO₂ emissions flux during the whole growth period of high water W₁ (90 mm) was significantly higher than that of low water W_0.7 (63 mm); the difference of soil CO₂ emissions between medium water level W_0.85 (76.5 mm) and low water level was not significant under medium and low nutrient condition F_0.85 (N 255 kg·hm^-2, P₂O₅ 76.5 kg·hm^-2). Soil CO₂ emissions intensity of high fertilizer F₁ was significantly larger (by 14.82%) than that of low fertilizer F_0.7 under high water supply W₁ (P<0.05), and that of F_0.85 was significantly bigger (by 8.03%) than that of F_0.7 in the medium water supply(P<0.05), while the difference between treatments at low water level was not significant(P>0.05). Compared with nonfertilized treatment, soil CO₂ cumulative emissions of the whole growth period with application of nitrogen fertilizer (210 kg·hm^-2), phosphate fertilizer (63 kg·hm^-2) and nitrogen mixed with phosphate fertilizer (210 kg·hm^-2, 63 kg·hm^-2) were significantly increased by 23.70%, 19.00% and 12.30%, respectively. And interaction effects between nitrogen and phosphorus fertilizer were extremely significant (P<0.01). Variance analysis of the interaction of water and fertilizer showed that the average soil CO₂ fluxes of the whole growth period were not statistically significant but cumulative emissions of soil CO₂ were significant (P<0.05) when the difference of supply level was 15%. In addition, soil CO₂ flux and cumulative emission in the whole growth period were significant when supply differed by 30%. Obviously, soil CO₂ emissions were promoted significantly by application of irrigation amount, nitrogen fertilizer, phosphorus rate and water and fertilizer interaction, while it was inhibited by nitrogen mixed with phosphorus. It was effective to regulate soil CO₂ emission by water and fertilizer controlling measures.

[Change of Soil Nitrogen Leaching with Summer Maize Growing Periods Under Plastic Film Mulched Cultivation in Danjiangkou Reservoir Area, China]

As an important form of agricultural non-point source pollution, soil nitrogen leaching deteriorates water quality. Compared with non-mulching cultivated land, field experiment explored the change characteristics of soil nitrogen leaching under plastic film mulching ridge-furrow in Wulongchi small watershed during summer maize growing period. The results showed that the amounts of mulching tillage soil TN and NO₃^--N leaching were significantly lower than those with non-mulched treatment, by 25.68% and 20.25%, respectively. With the advance of the summer maize growth period, leaching amount of mulched soil TN was highest at seedling stage, lowest at heading stage and higher in maturation period; leaching amount of mulched soil NO₃^--N was highest at seedling stage, lowest in maturation period; leaching amount of mulched soil NH₄⁺-N was lower at seedling stage, increased to the peak at the jointing stage, decreased to the valley value at heading stage, and obviously increased in maturation period. Linear function relationship was found between mulched soil TN leaching and TN content, while exponential relationship was found between mulched soil NO₃^--N leaching and NO₃^--N content. In addition, there was linear function relationship of mulched soil TN and NO₃^--N leaching amount with soil moisture and rainfall. It was concluded that the plastic film mulched on summer maize could reduce the leaching loss of soil nitrogen, and it would have a significant effect on the reduction of reservoir area of agricultural non-point source pollution.

[Assessment of Gaseous Nitrogen (NH3 and N2O) Mitigation After the Application of a Range of New Nitrogen Fertilizers in Summer Maize Cultivation]

In order to evaluate the potential of a range of new nitrogen fertilizers in comparison with the conventional fertilization to mitigate ammonia (NH₃) and nitrous oxide (N₂O) emissions, a field experiment was conducted to investigate NH₃ volatilization and N₂O emissions from the summer maize field and the relevant driving factors under the different nitrogen fertilizer treatments. Five new varieties of nitrogen fertilizers including the urea ammonium (UA), stability urea with dicyandiamide and hydroquinone (UHD), sulfur coated urea (SCU), urea formaldehyde compound fertilizer (UF) and organic fertilizer (OF) were applied in this experiment, and conventional fertilization (compound fertilizer + urea, CK) was used as the control. The nitrogen amount of 300 kg·hm^-2 was applied in all treatments. Correlation analysis results showed that both NH₃ volatilization and N₂O emissions were influenced by environmental factors. They were negatively correlated with soil water-filled pore space (P<0.05). Moreover, N₂O emissions were positively correlated with soil nitrate nitrogen (P<0.01). Regression analysis showed that N₂O emissions were mainly determined by the soil nitrate content, while NH₃ volatilization was mainly dependent on the values of soil ammonium nitrogen. Compared with CK, in addition to UA, other fertilizer treatments decreased the NH₃ volatilization, especially the UF and OF treatments decreased NH₃ volatilization by up to 37%-43%, while all treatments had no significant difference in N₂O emissions. Considering the total gaseous nitrogen losses (NH₃ volatilization + N₂O emissions), in comparison with CK, the UHD, SCU, UF and OF were reduced by 9%, 5%, 30% and 23%, respectively, while the UA was increased by 3%. Therefore, considering environmental benefit under this experimental condition,urea formaldehyde compound fertilizer and organic fertilizer were more suitable for maize cultivation.

[Effects of different cultivation patterns on yield, nitrate accumulation and nitrogen balance in winter wheat and summer maize rotation system.]

This study investigated the impacts of four cultivation patterns including farmer practice, high yield and high efficiency practice, super high yield practice, and super high yield and high efficiency practice on yields, soil nitrate and nitrogen (N) balances in 3 winter wheat-summer maize rotations with straw returning in Hebei Province. Results showed that the super high yield practice was identified with greatest winter wheat and summer maize yields, followed by high yield and high efficiency practice, and super high yield and high efficiency practice, which were all greater than that of farmer practice. The N use efficiency of high yield and high efficiency practice was significantly greater than the other cultivation patterns. The total nitrate accumulation in 0-400 cm soil of these cultivation patterns reached 768.4-1133.3 kg·hm^-2, where 80%-85% of the accumulated nitrate were in 90-400 cm soil. Meanwhile, the nitrate leaching was observed in all cultivation patterns and nitrate accumulation peaks at 120-150 cm and 270-330 cm were found. Soil nitrate content of high yield and high efficiency practice was less than 30 mg·kg^-1 and generally lower than other cultivation patterns, which to some extent reduced the environmental risk. In addition, nitrate surplus in 0-90 cm soil during winter wheat season was lower than that during summer maize season, and the high yield and high efficiency practice had the lowest apparent nitrogen loss. Overall, the high yield and high efficiency practice was evaluated to be the best cultivation pattern in consi-deration of yield, nitrogen use efficiency and nitrate accumulation in soil, but there was still certain achievable improvement potential.

[Soil moisture estimation method based on both ground-based remote sensing data and air temperature in a summer maize ecosystem.]

Soil moisture is an important component of the soil-vegetation-atmosphere continuum (SPAC). It is a key factor to determine the water status of terrestrial ecosystems, and is also the main source of water supply for crops. In order to estimate soil moisture at different soil depths at a station scale, based on the energy balance equation and the water deficit index (WDI), a soil moisture estimation model was established in terms of the remote sensing data (the normalized difference vegetation index and surface temperature) and air temperature. The soil moisture estimation model was validated based on the data from the drought process experiment of summer maize (Zea mays) responding to different irrigation treatments carried out during 2014 at Gucheng eco-agrometeorological experimental station of China Meteorological Administration. The results indicated that the soil moisture estimation model developed in this paper was able to evaluate soil relative humidity at different soil depths in the summer maize field, and the hypothesis was reasonable that evapotranspiration deficit ratio (i.e., WDI) linearly depended on soil relative humidity. It showed that the estimation accuracy of 0-10 cm surface soil moisture was the highest (R²=0.90). The RMAEs of the estimated and measured soil relative humidity in deeper soil layers (up to 50 cm) were less than 15% and the RMSEs were less than 20%. The research could provide reference for drought monitoring and irrigation management.

[Influences of micro-irrigation and subsoiling before planting on enzyme activity in soil rhizosphere and summer maize yield.]

In order to explore the influences of micro-irrigation and subsoiling before planting on enzyme activity in soil rhizosphere and summer maize yield, an orthogonal experiment was carried out with three factors of micro-irrigation method, irrigation depth, and subsoiling depth. The factor of irrigation method included surface drip irrigation, subsurface drip irrigation, and moistube-irrigation; three levels of irrigation depth were obtained by controlling the lower limit of soil water content to 50%, 65%, and 80% of field holding capacity, respectively; and three depths of deep subsoiling were 20, 40, and 60 cm. The results showed that the activities of catalase and urease increased first and then decreased, while the activity of phosphatase followed an opposite trend in the growth season of summer maize. Compared with surface drip irrigation and moistube-irrigation, subsurface drip irrigation increased the average soil moisture of 0-80 cm layer by 6.3% and 1.8% in the growth season, respectively. Subsurface drip irrigation could significantly increase soil urease activity, roots volume, and yield of summer maize. With the increase of irrigation level, soil phosphatase activity decreased first and then increased, while urease activity and yield increased first and then decreased. The average soil moisture and root volume all increased in the growth season of summer maize. The increments of yield and root volume from subsoiling of 40 to 20 cm were greater than those from 60 to 40 cm. The highest enzyme activity was obtained with the treatment of subsoiling of 40 cm. In terms of improving water resource use efficiency, nitrogen use efficiency, and crop yield, the best management strategy of summer maize was the combination of subsurface drip irrigation, controlling the lower limit of soil water content to 65% of field holding capacity, and 40 cm subsoiling before planting.

[Effects of nitrapyrin-nitrogen (N) fertilizer application rates on N utilization and N2O emission in summer maize field]

To reduce the N₂O emission from soil and enhance N utilization by crop, a field experiment was carried out to study the effects of nitrapyrin-N fertilizer application rates (0, 180, 270, 360 kg N·hm^-2) on soil N₂O emission and N apparent loss, grain yield and N utilization of summer maize. Results showed that the soil N₂O emission under different N fertilizer treatments mainly occurred in periods from sowing to seedling, and from jointing to tasseling. Soil N₂O emission peaks were observed after basal and top dressing events. Maize yield increased with N fertilizer rates but there was no significant difference between 270 and 360 kg N·hm^-2, and the net income of these two treatments was 5209 and 5426 yuan·hm^-2, respectively. Compared with no N fertilizer treatment, the N uptake in the N fertilizer treatments was increased by 109.6%-134.1%. The treatment of 270 kg N·hm^-2 had the highest agronomic N efficiency and N use efficiency, but the N apparent loss was low. The treatment with nitrapyrin-N fertilization rate of 270 kg N·hm^-2 appeared to be the optimal rate to obtain high maize yield and N use efficiency, and low soil N₂O emission and N apparent loss.