Assimilation of temporal-spatial leaf area index into the CERES-Wheat model with ensemble Kalman filter and uncertainty assessment for improving winter wheat yield estimation

To accurately estimate winter wheat yields and analyze the uncertainty in crop model data assimilations,winter wheat yield estimates were obtained by assimilating measured or remotely sensed leaf area index(LAI) values. The performances of the calibrated crop environment resource synthesis for wheat(CERES-Wheat) model for two different assimilation scenarios were compared by employing ensemble Kalman filter(EnKF)-based strategies. The uncertainty factors of the crop model data assimilation was analyzed by considering the observation errors,assimilation stages and temporal-spatial scales. Overall,the results indicated a better yield estimate performance when the EnKF-based strategy was used to comprehensively consider several factors in the initial conditions and observations. When using this strategy,an adjusted coefficients of determination(R~2) of 0.84,a root mean square error(RMSE) of 323 kg ha~(–1),and a relative errors(RE) of 4.15% were obtained at the field plot scale and an R~2 of 0.81,an RMSE of 362 kg ha~(–1),and an RE of 4.52% were obtained at the pixel scale of 30 m×30 m. With increasing observation errors,the accuracy of the yield estimates obviously decreased,but an acceptable estimate was observed when the observation errors were within 20%. Winter wheat yield estimates could be improved significantly by assimilating observations from the middle to the end of the crop growing seasons. With decreasing assimilation frequency and pixel resolution,the accuracy of the crop yield estimates decreased; however,the computation time decreased. It is important to consider reasonable temporal-spatial scales and assimilation stages to obtain tradeoffs between accuracy and computation time,especially in operational systems used for regional crop yield estimates.     

Contribution of Drought to Potential Crop Yield Reduction in a Wheat-Maize Rotation Region in the North China Plain

With consecutive occurrences of drought disasters in China in recent years, it is important to estimate their potential impacts on regional crop production. In this study, we detect the impacts of drought on wheat and maize yield and their changes at a 0.5＆#176;＆#215;0.5＆#176; grid scale in the wheat-maize rotation planting area in the North China Plain under the A1B climate change scenario using the Decision Support System for Agrotechnology Transfer （DSSAT） model and the outputs of the regional climate modeling system-Providing Regional Climates for Impacts Studies （PRECIS）. Self-calibrating palmer drought severity index was used as drought recognition indicator. Two time slices used for the study were the baseline （1961-1990） and 40 years of 2011-2050. The results indicate that the potential planting region for double crop system of wheat-maize would expend northward. The statistic conclusions of crop simulations varied considerably between wheat and maize. In disaster-affected seasons, wheat yield would increase in the future compared with baseline yields, whereas in opposite for maize yield. Potential crop yield reductions caused by drought would be lower for wheat and higher for maize, with a similar trend found for the ratio of potential crop yield reductions for both crops. It appears that the negative impact of drought on maize was larger than that on wheat under climate change A1B scenario.     

Suitability of the DNDC model to simulate yield production and nitrogen uptake for maize and soybean intercropping in the North China Plain

Intercropping is an important agronomic practice. However, assessment of intercropping systems using field experiments is often limited by time and cost. In this study, the suitability of using the DeNitrification DeComposition(DNDC) model to simulate intercropping of maize(Zea mays L.) and soybean(Glycine max L.) and its aftereffect on the succeeding wheat(Triticum aestivum L.) crop was tested in the North China Plain. First, the model was calibrated and corroborated to simulate crop yield and nitrogen(N) uptake based on a field experiment with a typical double cropping system. With a wheat crop in winter, the experiment included five treatments in summer: maize monoculture, soybean monoculture, intercropping of maize and soybean with no N topdressing to maize(N0), intercropping of maize and soybean with 75 kg N ha~(–1) topdressing to maize(N75), and intercropping of maize and soybean with 180 kg N ha~(–1) topdressing to maize(N180). All treatments had 45 kg N ha~(–1) as basal fertilizer. After calibration and corroboration, DNDC was used to simulate long-term(1955 to 2012) treatment effects on yield. Results showed that DNDC could stringently capture the yield and N uptake of the intercropping system under all N management scenarios, though it tended to underestimate wheat yield and N uptake under N0 and N75. Long-term simulation results showed that N75 led to the highest maize and soybean yields per unit planting area among all treatments, increasing maize yield by 59% and soybean yield by 24%, resulting in a land utilization rate 42% higher than monoculture. The results suggest a high potential to promote soybean production by intercropping soybean with maize in the North China Plain, which will help to meet the large national demand for soybean.     

Developing sustainable summer maize production for smallholder farmers in the North China Plain: An agronomic diagnosis method

With an increasing population and changing diet structure, summer maize is increasingly becoming an important energy crop in China. However, traditional farmer practices for maize production are inefficient and unsustainable. To ensure food security and sustainable development of summer maize production in China, an improved, more sustainable farmer management system is needed. Establishing this system requires a comprehensive understanding of the limitations of current farming practice and the ways it could be improved. In our study, 235 plots from three villages in the North China Plain(NCP) were monitored. Maize production on farms was evaluated; our results showed that the maize yield and nitrogen partial factor productivity(PFP_N) were variable on smallholder farms at 6.6~(–1)3.7 t ha~(–1) and 15.4–88.7 kg kg~(–1), respectively.Traditional farming practices also have a large environmental impact(nitrogen surplus: –64.2–323.78 kg ha~(–1)). Key yield components were identified by agronomic diagnosis. Grain yield depend heavily on grain numbers per hectare rather than on the 1 000-grain weight. A set of improved management practices(IP) for maize production was designed by employing a boundary line(BL) approach and tested on farms. Results showed that the IP could increase yield by 18.4% and PFP_N by 31.1%, compared with traditional farmer practices(FP), and reduce the nitrogen(N) surplus by 57.9 kg ha~(–1). However,in terms of IP effect, there was a large heterogeneity among different smallholder farmers' fields, meaning that, precise technologies were needed in different sites especially for N fertilizer management. Our results are valuable for policymakers and smallholder farmers for meeting the objectives of green development in agricultural production.     

Soil mineral nitrogen and yield-scaled soil N_2O emissions lowered by reducing nitrogen application and intercropping with soybean for sweet maize production in southern China

The increasing demand for fresh sweet maize(Zea mays L.saccharata)in southern China has prioritized the need to find solutions to the environmental pollution caused by its continuous production and high inputs of chemical nitrogen fertilizers.A promising method for improving crop production and environmental conditions is to intercrop sweet maize with legumes.Here,a three-year field experiment was conducted to assess the influence of four different cropping systems(sole sweet maize(SS),sole soybean(SB),two rows sweet maize-three rows soybean(S2B3)intercropping,and two rows sweet maize-four rows soybean(S2B4)intercropping),together with two rates of N fertilizer application(300 and 360 kg N ha~(–1))on grain yield,residual soil mineral N,and soil N_2O emissions in southern China.Results showed that in most case,intercropping achieved yield advantages(total land equivalent ratio(TLER=0.87–1.25)was above one).Moreover,intercropping resulted in 39.8%less soil mineral N than SS at the time of crop harvest,averaged over six seasons(spring and autumn in each of the three years of the field experiment).Generally,intercropping and reduced-N application(300 kg N ha~(–1))produced lower cumulative soil N_2O and yield-scaled soil N_2O emissions than SS and conventional-N application(360 kg N ha~(–1)),respectively.S2B4 intercropping with reduced-N rate(300 kg N ha~(–1))showed the lowest cumulative soil N_2O(mean value=0.61 kg ha~(–1))and yield-scaled soil N_2O(mean value=0.04 kg t~(–1))emissions.Overall,intercropping with reduced-N rate maintained sweet maize production,while also reducing environmental impacts.The system of S2B4 intercropping with reduced-N rate may be the most sustainable and environmentally friendly cropping system.     

Top-grain filling characteristics at an early stage of maize(Zea mays L.) with different nitrogen use efficiencies

Maize genotypes vary significantly in their nitrogen use efficiencies(NUEs).Better understanding of early grain filling characteristics of maize is important,especially for maize with different NUEs.The objectives of this research were(i)to investigate the difference in apical kernel development of maize with different NUEs,(ii)to determine the reaction of apical kernel development to N application levels,and(iii)to evaluate the relationship between apical kernel development and grain yield(GY)for different genotypes of maize.Three maize hybrid varieties with different NUEs were cultivated in a field with different levels of N fertilizer arranged during two growing seasons.Kernel fresh weight(KFW),volume(KV)and dry weight(KDW)of apical kernel were evaluated at an early grain filling stage.Ear characteristics,GY and its components were determined at maturity stage.Apical kernel of the high N and high efficiency(HN-HE)type(under low N,the yield is lower,and under higher N,the yield is higher)developed better under high N(N210 and N240,pure N of 210 and 240 kg ha~(–1))than at low N(N120 and N140,pure N of 120 and 140 kg ha~(–1)).The low N and high efficiency(LN-HE)type(under low N,the yield is higher,while under higher N,the yield is not significantly higher)developed better under low N than at high N.The double high efficiency(D-HE)type(for both low and high N,the yield is higher)performed well under both high and low N.Apical kernel reacted differently to the N supply.Apical kernel developed well at an early grain filling stage and resulted in a higher kernel number(KN),kernel weight(KW)and GY with better ear characteristics at maturity.     

Observed Climatic Variations in the Growing Season of Field Crops in Northeast China from 1992 to 2012

To determine the potential effects of climate change on crop phenological development and productivity, an integrated analysis was conducted based on the observed climatic and phenological records of Northeast China from 1992 to 2012. A set of quality assurance procedures, including repeated record checks, agro-meteorological station selection, internal consistency checks, temporal outlier checks, spatial outlier checks, and interpolation of missing data, were designed and applied to the phenology datasets of spring maize and paddy rice. Our results indicated that almost all phenological dates of spring maize and paddy rice became increasingly delayed from 1992 to 2012, The duration of the growing season was prolonged, particularly for the grain-filling stage （GS3）. The prolonged growing season was beneficial to productivity. For spring maize, the average precipitation during GS3 decreased at a rate of 27.46 mm/decade, and the annual accumulated temperature over 10℃ increased at a rate of 31.07℃/ decade. Farmers initiatively adjusted crop cultivars and selected drought-resistant crops to cope with the challenges of drought.     

Integrated agronomic practices management improved grain formation and regulated endogenous hormone balance in summer maize(Zea mays L.)

Compared with single agronomic practices management during grain formation, knowledge about integrated agronomic practices management on grain-filling characteristics and physiological function of endogenous hormones was limited. In order to clarify this issue, two field experiments, integrated agronomic practices management(IAPM), T1(local conventional cultivation practices), T2(an optimized combination of cropping systems and fertilizer treatment), T3(treatment based on high-yield studies), and T4(further optimized combination of cropping systems and fertilizer treatment), and nitrogen rate testing(NAT)(four nitrogen rates, 0, 129.0, 184.5, and 300.0 kg N ha–1) were performed with summer maize hybrid Zhengdan 958(ZD958). Results showed that with increased nitrogen rate, the endogenous hormone balance was promoted and the grain-filling characteristics were improved sufficiently to resulting in a significant increase in grain yield. However, the grain-filling characteristics deteriorated and yield was reduced with excessive nitrogen fertilization. However, IAPM could promote hormone balance and improve grain filling characteristic. The indole-3-acetic acid(IAA), zeatin riboside(ZR), and gibberellin(GA3) contents under T2 and T4 treatments were higher and the abscisic acid(ABA) content was lower, and the ZR and GA3 contents under T3 were higher than those under T1. Those resulted in the maximum grain-filling rate(Wmax) and the active grain-filling period(P) under T2, T3 and T4 were significantly increased than those under T1, and hence promoted kernel weight and grain yield. So IAPM promoted hormone balance by improving tillage model, optimizing fertilizer rate and fertilization period, appropriately increasing planting density and delaying harvest, which promoted grain filling rate and lengthened active grain-filling period, finally increased grain yield.     

Yield gap and resource utilization efficiency of three major food crops in the world——A review

Yield gap analysis could provide management suggestions to increase crop yields,while the understandings of resource utilization efficiency could help judge the rationality of the management.Based on more than 110 published papers and data from Food and Agriculture Organization (FAO,www.fao.org/faostat) and the Global Yield Gap and Water Productivity Atlas (www.yieldgap.org),this study summarized the concept,quantitative method of yield gap,yield-limiting factors,and resource utilization efficiency of the three major food crops (wheat,maize and rice).Currently,global potential yields of wheat,maize and rice were 7.7,10.4 and 8.5 t ha~(–1),respectively.However,actual yields of wheat,maize and rice were just 4.1,5.5 and 4.0 t ha~(–1),respectively.Climate,nutrients,moisture,crop varieties,planting dates,and socioeconomic conditions are the most mentioned yield-limiting factors.In terms of resource utilization,nitrogen utilization,water utilization,and radiation utilization efficiencies are still not optimal,and this review has summarized the main improvement measures.The current research focuses on quantitative potential yield and yield gap,with a rough explanation of yield-limiting factors.Subsequent research should use remote sensing data to improve the accuracy of the regional scale and use machine learning to quantify the role of yield-limiting factors in yield gaps and the impact of change crop management on resource utilization efficiency,so as to propose reasonable and effective measures to close yield gaps.     

Comparative effects of nitrogen application on growth and nitrogen use in a winter wheat/summer maize rotation system

The application of fertilizer in agricultural production has become universally common for achieving high crop yields and economic benefits, but it has potential impacts on food safety, energy crisis and environmental pollution. Optimal management of fertilization is thus necessary for maintaining sustainable agriculture. Two-year(2013–2015) field experiment was conducted, in Yangling(108°24′E, 34°20′N, and 521 m a.s.l.), Shaanxi Province, China, to explore the effects of different nitrogen(N) applications on biomass accumulation, crop N uptake, nitrate N(NO_3~–-N) distribution, yield, and N use with a winter wheat/summer maize rotation system. The N applications consisted of conventional urea(U)(at 80(U80), 160(U160), and 240(U240) kg N ha~(–1); 40% applied as a basal fertilizer and 60% top-dressed at jointing stage) and controlled-release urea(CRU)(at 60(C60), 120(C120), 180(C180), and 240(C240) kg N ha~(~(–1)); all applied as a basal fertilizer) with no N application as a control(CK). The continuous release of N from CRU matched well with the N demands of crop throughout entire growing stages. Soil NO_3~–-N content varied less and peaked shallower in CRU than that in urea treatments. The differences, however, were smaller in winter wheat than that in summer maize seasons. The average yield of summer maize was the highest in C120 in CRU treatments and in U160 in urea treatments, and apparent N use efficiency(NUE) and N agronomic efficiency(NAE) were higher in C120 than in U160 by averages of 22.67 and 41.91%, respectively. The average yield of winter wheat was the highest in C180 in CRU treatments and in U240 in urea treatments with C180 increasing NUE and NAE by averages of 14.89 and 35.62% over U240, respectively. The annual yields under the two N fertilizers were the highest in C120 and U160. The results suggested that CRU as a basal fertilizer once could be a promising alternative of urea as split application in semiarid areas.     

Impact of climate change on maize yield in China from 1979 to 2016

Climate change severely impacts agricultural production, which jeopardizes food security. China is the second largest maize producer in the world and also the largest consumer of maize. Analyzing the impact of climate change on maize yields can provide effective guidance to national and international economics and politics. Panel models are unable to determine the group-wise heteroscedasticity, cross-sectional correlation and autocorrelation of datasets, therefore we adopted the feasible generalized least square(FGLS) model to evaluate the impact of climate change on maize yields in China from 1979–2016 and got the following results:(1) During the 1979–2016 period, increases in temperature negatively impacted the maize yield of China. For every 1°C increase in temperature, the maize yield was reduced by 5.19 kg 667 m–2(1.7%). Precipitation increased only marginally during this time, and therefore its impact on the maize yield was negligible. For every 1 mm increase in precipitation, the maize yield increased by an insignificant amount of 0.043 kg 667 m–2(0.014%).(2) The impacts of climate change on maize yield differ spatially, with more significant impacts experienced in southern China. In this region, a 1°C increase in temperature resulted in a 7.49 kg 667 m–2 decrease in the maize yield, while the impact of temperature on the maize yield in northern China was insignificant. For every 1 mm increase in precipitation, the maize yield increased by 0.013 kg 667 m–2 in southern China and 0.066 kg 667 m–2 in northern China.(3) The resilience of the maize crop to climate change is strong. The marginal effect of temperature in both southern and northern China during the 1990–2016 period was smaller than that for the 1979–2016 period.     

Screening of antagonistic Trichoderma strains and their application for controlling stalk rot in maize

Maize is one of the major crops in China, but maize stalk rot occurs nationwide and has become one of the major challenges in maize production in China. In order to find an environment-friendly and feasible technology to control this disease, a Trichoderma-based biocontrol agent was selected. Forty-eight strains with various inhibition activities to Fusarium graminearum, and Fusarium verticillioides were tested. A group of Trichoderma strains(DLY31, SG3403, DLY1303 and GDFS1009) were found to provide an inhibition rate to pathogen growth in vitro of over 70%. These strains also prevented pathogen infection over 65% and promoted the maize seedling growth for the main root in vivo by over 50%. Due to its advantage in antifungal activity against pathogens and promotion activity to maize, Trichoderma asperellum GDSF1009 was selected as the most promising strain of the biocontrol agent in the Trichoderma spectrum. Pot experiments showed that the Trichoderma agent at 2–3 g/pot could achieve the best control of seedling stalk rot and promotion of maize seedling growth. In the field experiments, 8–10 g/hole was able to achieve over 65% control to stalk rot, and yield increased by 2–11%. In the case of natural morbidity, the control efficiency ranged from 27.23 to 48.84%, and the rate of yield increase reached 11.70%, with a dosage of Trichoderma granules at 75 kg ha~(-1). Based on these results, we concluded that the Trichoderma agent is a promising biocontrol approach to stalk rot in maize.     

Effects of Selenium on Maize Ovary Development at Pollination Stage Under Water Deficits

Maize ovary development is linked to kernel formation. Soil water deficit results in ovary abortion because of low water potential （ψw） resulting in inhibition of photosynthesis from anthesis to silking stage. Thus, drought is a key factor causing yield losses in maize, especially near the time of pollination. Earlier studies have indicated that selenium （Se） maintains antioxidative defence systems and enhances sugar and starch accumulation. The effects of Se on maize ovary development were studied in drought stress conditions. Maize ovary development was studied through observations on starch, membrane integrity, fresh weight and dry weight in drought-treated or unstressed glasshouse-grown plants, and crop yield, yield structure, leaf, stem and ear biomass accumulation were also analyzed. Results indicated that Se increased the stress tolerance of the crop, even though ovary abortion was not eliminated by Se treatments under low ψw. Under well-watered control conditions, Se had also negative effects. When the first ear was not succesfully pollinated or the ovaries aborted, the plants developed more ears. Even though these later formed ears did not produce kernels, they significantly increased the dry weight of the plants under water deficit. It could be concluded that The optimal concentration for maize of Se applied through roots is about 0.01 mg kg^-1 （supplied as Na2SeO4） soil or less.     

Cultivar selection can increase yield potential and resource use efficiency of spring maize to adapt to climate change in Northeast China

Northeast China (NEC) is one of the major maize production areas in China.Agro-climatic resources have obviously changed,which will seriously affect crop growth and development in this region.It is important to investigate the contribution of climate change adaptation measures to the yield and resource use efficiency to improve our understanding of how we can effectively ensure high yield and high efficiency in the future.In this study,we divided the study area into five accumulated temperature zones (ATZs) based on growing degree days (GDD).Based on the meteorological data,maize data (from agrometeorological stations) and the validated APSIM-Maize Model,we first investigated the spatial distributions and temporal trends of maize potential yield of actual planted cultivars,and revealed the radiation use efficiency (RUE) and heat resource use efficiency (HUE) from 1981 to 2017.Then according to the potential growing seasons and actual growing seasons,we identified the utilization percentages of radiation (P_R) resource and heat resource (P_H) for each ATZ under potential production from 1981 to 2017.Finally,we quantified the contributions of cultivar changings to yield,P_R and P_H of maize.The results showed that during the past 37 years,the estimated mean potential yield of actual planted cultivars was 13 649 kg ha~(–1),ranged from 11 205 to 15 257 kg ha~(–1),and increased by 140 kg ha~(–1) per decade.For potential production,the mean values of RUE and HUE for the actual planted maize cultivars were 1.22 g MJ~(–1) and 8.58 kg (℃ d)~(–1) ha~(–1).RUE showed an increasing tendency,while HUE showed a decreasing tendency.The lengths of the potential growing season and actual growing season were 158 and 123 d,and increased by 2 and 1 d per decade.P_R and P_H under potential production were 82 and 86%,respectively and showed a decreasing tendency during the past 37 years.This indicates that actual planted cultivars failed to make full use of climate resources.However,results from the adaptation assessments indicate that,adoption of cultivars with growing season increased by 2–11 d among ATZs caused increase in yield,P_R and P_H of 0.6–1.7%,1.1–7.6% and 1.5–8.9%,respectively.Therefore,introduction of cultivars with longer growing season can effectively increase the radiation and heat utilization percentages and potential yield.     

Comparative Analysis of Genetic Diversity in Landraces of Waxy Maize from Yunnan and Guizhou Using SSR Markers

Waxy maize landraces are abundant inYunnan and Guizhou of China. Genetic diversity of waxy maize landraces from Yunnan and Guizhou were analyzed using SSR markers. We screened 38 landraces with 50 primers that generated 3 to 6 polymorphic bands, with an average of 4.13 bands. Shannon＇s information indices for genetic diversity of the 14 waxy maize landraces from Yunnan varied from 4.9571 to 42.1138 and averaged 26.5252; Shannon＇s information indices for genetic diversity of the 24 waxy maize landraces from Guizhou varied from 22.0066 to 40.6320 and averaged 32.3156. For the 14 waxy maize landraces from Yunnan, the within-landrace genetic diversity accounted for 45.40% and the among-landrace genetic diversity accounted for 54.60% of the total genetic diversity observed. For the 24 waxy maize landraces from Guizhou, the within-landrace genetic diversity accounted for 50.76% and the among-landrace genetic diversity accounted for 49.24% of the total observed. Some individual landraces possessed as much as 96.86% of the total genetic diversity occurring among landraces within origins. Differentiation between geographic origins accounted for only 3.14% of the total genetic diversity. Both Yunnan and Guizhou would be the diversity centers and the original centers of waxy maize.     

The Impact of US and CGIAR Germplasm on Maize Production in China

Wide adoption of a few kinds of homogeneous germplasm would reduce crop genetic diversity, increase crop vulnerability to stresses, and reduce the stability of crop production. The introduction and utilization of foreign germplasm is a sustainable solution for broadening the genetic diversity and promoting periodical replacement of varieties. The genetic contribution and economic impact of foreign germplasm, particularly those of US and CGIAR （Consultative Group on International Agricultural Research, referred to as the CG system） materials, on China＇s maize production are evaluated on the basis of an analysis of variety pedigree information from 20 major maize-producing provinces in China from 1982 to 1997. The results indicated that the contribution of US and CG germplasm to Chinese maize production continues to increase, particularly CG germplasm, which has shown a rapid increasing trend since the 1990s. If the genetic contribution of US germplasm is increased by 1%, maize yield will gain by 0.2% （0.01 t ha^-1）. If contribution of CG germplasm, which has greater production potential, is increased by 1%, maize yield will gain by 0.025 t ha^-1. A policy should be implicated by the government in this direction to encourage breeders to focus more on the use and improvement of CG germplasm. The US germplasm has been utilized extensively in China so that it can offer germplasm resources for maize breeding efforts.     

Straw return and appropriate tillage method improve grain yield and nitrogen efficiency of winter wheat

Straw return is an important management tool for tackling and promoting soil nutrient conservation and improving crop yield in Huang-Huai-Hai Plain, China. Although the incorporation of maize straw with deep plowing and rotary tillage practices are widespread in the region, only few studies have focused on rotation tillage. To determine the effects of maize straw return on the nitrogen(N) efficiency and grain yield of winter wheat(Triticum aestivum L.), we conducted experiments in this region for 3 years. Five treatments were tested:(i) rotary tillage without straw return(RT);(ii) deep plowing tillage without straw return(DT);(iii) rotary tillage with total straw return(RS);(iv) deep plowing tillage with total straw return(DS);(v) rotary tillage of 2 years and deep plowing tillage in the 3rd year with total straw return(TS). Treatments with straw return increased kernels no. ear~(–1), thousand-kernel weight(TKW), grain yields, ratio of dry matter accumulation post-anthesis, and nitrogen(N) efficiency whereas reduced the ears no. ha~(–1) in the 2011–2012 and 2012–2013 growing seasons. Compared with the rotary tillage, deep plowing tillage significantly increased the grain yield, yield components, total dry matter accumulation, and N efficiency in 2013–2014. RS had significantly higher straw N distribution, soil inorganic nitrogen content, and soil enzymes activities in the 0–10 cm soil layer compared with the DS and TS. However, significantly lower values were observed in the 10–20 and 20–30 cm soil layers. TS obtained approximately equal grain yield as DS, and it also reduced the resource costs. Therefore, we conclude that TS is the most economical method for increasing grain yield and N efficiency of winter wheat in Huang-Huai-Hai Plain.     

Soil mulching can mitigate soil water defi ciency impacts on rainfed maize production in semiarid environments

Temporally irregular rainfall distribution and inefficient rainwater management create severe constraints on crop production in rainfed semiarid areas. Gravel and plastic film mulching are effective methods for improving agricultural productivity and water utilization. However, the effects of these mulching practices on soil water supply and plant water use associated with crop yield are not well understood. A 3-yr study was conducted to analyze the occurrence and distribution of dry spells in a semiarid region of Northwest China and to evaluate the effects of non-mulching(CK), gravel mulching(GM) and plastic film mulching(FM) on the soil water supply, plant water use and maize(Zea mays L.) grain yield. Rainfall analysis showed that dry spells of ≥5 days occurred frequen tly in each of 3 yr, accounting for 59.9–69.2% of the maize growing periods. T he 15-d dry spells during the jointing stage would expose maize plants to particularly severe water stress. Compared with the CK treatment, both the GM and FM treatments markedly increased soil water storage during the early growing season. In general, the total evapotranspiration(ET) was not significantly different among the three treatments, but the mulched treatments significantly increased the ratio of pre- to post-silking ET, which was closely associated with yield improvement. As a result, the grain yield significantly increased by 17.1, 70.3 and 16.7% for the GM treatment and by 28.3, 87.6 and 38.2% for the FM treatment in 2010, 2011 and 2012, respectively, compared with the CK treatment. It's concluded that both GM and FM are effective strategies for mitigating the impacts of water deficit and improving maize production in semiarid areas. However, FM is more effective than GM.     

Responses of Nitrogen Uptake and Yield of Winter Wheat to Nonuniformity of Sprinkler Fertigation

Field experiments were conducted to investigate the effects of nonuniformity of sprinkler fertigation and the amount of fertilizers applied through fertigation on nitrogen uptake and crop yield during two growing seasons of winter wheat in 2002-2003 and 2003-2004 at an experimental station in Beijing. In the experiments, the seasonal averaged Christiansen irrigation uniformity coefficient （CU） varied from 72% to 84%. Except for the fertilizer applied before planting, fertilizer was applied with the sprinkler irrigation system with a seasonal averaged CU for fertigation varied from 71% to 85%. Three levels of fertilizer applied varying from 0 to 180 kg N ha^-1 were used in the experiments. The experimental results demonstrated that sprinkler fertigation uniformity had insignificant effects on nitrogen uptake and crop yield for the uniformity range tested. Also, the influence of fertilizer applied through sprinkler fertigation on crop yield was minor, while the total nitrogen content for stem and nitrogen uptake increased with increasing fertilizer applied.     

Effects of long-term full straw return on yield and potassium response in wheat-maize rotation

The effect of long-term straw return on crop yield, soil potassium(K) content, soil organic matter, and crop response to K from both straw and chemical K fertilizer(K_2SO_4) were investigated in a fixed site field experiment for winter wheat-summer maize rotation in 6 years for 12 seasons. The field experiment was located in northern part of North China Plain with a sandy soil in relatively low yield potential. Two factors, straw return and chemical K fertilizer, were studied with two levels in each factor. Field split design was employed, with two straw treatments, full straw return of previous crop(St) and no straw return, in main plots, and two chemical K fertilizer treatments, 0 and 60 kg K2 O ha~(–1), as sub-plots. The results showed that straw return significantly increased yields of winter wheat and summer maize by 16.5 and 13.2% in average, respectively, and the positive effect of straw return to crop yield showed more effective in lower yield season. Straw return significantly increased K absorption by the crops, with significant increase in straw part. In treatment with straw return, the K content in crop straw increased by 15.9 and 21.8% in wheat and maize, respectively, compared with no straw return treatment. But, straw return had little effect on K content in grain of the crops. Straw return had significant influences on total K uptake by wheat and maize plants, with an increase of 32.7 and 30.9%, respectively. There was a significant correlation between crop yield and K uptake by the plant. To produce 100 kg grain, the wheat and maize plants absorbed 3.26 and 2.24 kg K2 O, respectively. The contents of soil available K and soil organic matter were significantly affected by the straw return with an increase of 6.07 and 23.0%, respectively, compared to no straw return treatment. K_2SO_4 application in rate of 60 kg K2 O ha~(–1) showed no significant effect on wheat and maize yield, K content in crop straw, total K uptake by the crops, soil available K content, and soil organic matter. The apparent K utilization rate(percentage of applied K absorbed by the crop in the season) showed difference for wheat and maize with different K sources. In wheat season, the K utilization rate from K_2SO_4 was higher than that from straw, while in maize season, the K utilization rate from straw was higher than that from chemical fertilizer. In the whole wheat-maize rotation system, the K absorption efficiency by the two crops from straw was higher than that from K_2SO_4.