Management And Regulation Of Water And Nitrogen For High-yield And High-efficiency Dryland Maize System

In China, approximately70%of the national territory area is dryland regions. Therainfed farmland area is about500million hectares, accounting for50%of the total farmlandarea. Therefore, dryland agriculture is essential for ensuring national food security. Maize(Zea mays L.) is one of the dominate grain crops in dryland areas. However, due to the limitedwater resource, the maize grain yield is always maintained at a low level, and there are greatopportunities to further enhance the yield by closing the yield gap. In addition, overuse ofchemical fertilizers is a common problem in the dryland maize production system, resulting indecreased nitrogen (N) use efficiency, increased N losses and the risk of environmentalpollution. For these problems, we proposed three hypotheses:(1) the rainfall during maizegrowth season can satisfy maize water demand to achieve the climate potential productivity,and frequent and long dry spell is the major factor resulting in water stress and yield loss inthe semiarid south region of the Loess Plateau;(2) under plastic film mulching conditions,increasing plant density can markedly improve maize yield and water use efficiency;(3)understanding plant N accumulation dynamics and determining the rational N application rateare essential requirements to increase maize yield, water and N ues efficiency, and decreasesoil nitrate nitrogen (NO3-N) leaching and/or build up, and reduce nitrous oxide (N2O)emissions as well. To test these hypotheses, we conducted two field experiments in Changwucounty of Shaanxi province over three years, which evaluated the effects of differentmulching practices and N application rates on soil moisture and mineral N content, maizegrain yield, dry matter production and aboveground N accumulation, water and N useefficiency and N2O emissions. The main results of this study were concluded as follows:(1) The poor rainfall distribution over time and inefficient rainwater managementrather than the total amount of rainfall were the major factors, causing water stress andlimiting dryland maize production. The cumulative length of≥5days dry spell (a dry day is aday with rainfall <0.85mm and a dry spell is any consecutive number of dry days) occupiedexceeding60%of the maize growing periods, most of which distributed during the maizevegetative stage. Gravel and plastic film mulching significantly increased the soil waterstorage during the early maize growth seasons. The total evapotranspiration (ET) throughout the entire maize growth season showed no significant difference among the three treatments,but the two mulched treatments significantly increased the ratio of pre-to post-silking ET.This implied that soil mulching not only increased soil water availability, but also ensuredplant water use during dry seasons. As a result, the water use efficiency for the gravel-andfilm-mulched treatment was significantly increased by14.8-51.0%and25.6-88.0%,respectively, and the grain yield was significantly increased by16.7-70.3%and28.3-87.6%,respectively, compared to the non-mulched treatment.(2) Gravel and plastic film mulching markedly increased the aboveground Naccumulation during the entire growth season. The total N accumulation at harvest was higherby12.8-41.2%and33.2-55.8%in the gravel-and film-mulched treatments than that in thenon-mulched treatment, respectively, implying higher N uptake efficiency. The N harvestindex (NHI) showed no significant difference between the two mulched treatments, but thesevalues were significantly increased by9.6-32.4%compared with the non-mulched treatment,implying higher N utilization efficiency, and this was most likely attributed to the higher Ntranslocation and N accumulation post-silking in the mulched treatments. Overall, the N useefficiency was significantly increased by16.7-70.3%and28.3-87.6%for the gravel-andfilm-mulched treatments, respectively. In addition, soil NO3-N leaching occurred in the wetyears (2010and2011) and NO3-N buildup in the0-100cm layer was observed in the dry year(2012) for all the treatments. However, both the NO3-N leaching and buildup were clearlymitigated in the mulching treatments.(3) Under plastic film mulching conditions, increasing the plant density from65,000to85,000plants ha-1dramatically increased dryland maize grain yield, theenhancements were7.5-9.8%. This was probably because that high density system improvedthe utilization efficiency of the retained soil water, and then significantly increased theharvested ear numbers, though the kernel numbers per ear was decreased at the same time.Film mulching for the whole growing season promoted maize growth and development duringthe reproductive stage. Removing the film at the silking stage decreased the topsoiltemperature, thereafter decreased plant leaf and root senescence rate, extended the duration ofthe grain-filling period (3-4days) and increased the kernel numbers per ear and1000-kernelweight at harvest. Finally, the grain yield was further increased by0.6-1.2t ha-1.(4) N fertilization significantly affected dryland maize grain yield, dry matter and Naccumulation. When the N rate was below250kg ha-1, the grain yield increased significantlyas the N fertilizer rate increased. This trend was also confirmed by dry matter and Naccumulation. Stabilized high grain yields (13.1-15.1t ha-1) were obtained using N fertilizerrates of200-400kg ha-1in the years despite strong year-to-year differences in rainfall. There was a linear-plateau relationship between the grain yield and the dry matter and Naccumulation during the pre-silking stage, and a linear relationship during the post-silkingstage. This implies that optimizing N management to improve dry matter and N accumulation(especially post-silking) is the key to ensuring a high yield of film-mulched maize.(5) The apparent N losses during the maize growth season and the residual soilNO3-N contents in the0-100cm layer after harvest markedly increased with increased Nfertilizer rate. Excessive N applications resulted in higher NO3-N leaching in the wet years.The N use efficiency decreased with increased N fertilizer rate, but insignificant differences inagronomic efficiency, apparent recovery efficiency and physiological efficiency wereobserved between N fertilizer rates of100,200and250kg ha-1. A linear-plateau relationshipwas observed between grain yield and N supply (fertilizer N plus initial soil NO3-N in the0-100cm layers). The minimal N supply rate required to obtain the maximum yield (13.9tha-1) was279kg N ha-1, which was similar to plant N uptake and simultaneously reducedapparent N losses. This implies that N fertilizer recommendations which consider soil NO3-Ncontents can simultaneously provide sufficient N for high yields and improve N fertilizationof film mulched maize in dryland areas.(6) Gravel and plastic film mulching increased the topsoil temperature and moisture,but the N2O emissions showed no difference between the mulched and non-mulchedtreatments, most likely because the mulched treatments significantly increased the maize Nuptake and then decreased soil mineral N content, consequently limited the N2O production.However, due to the increased grain yield, the yield-scaled N2O emissions were markedlyreduced in the gravel-and film-mulched treatments, and a greater reduction was observed inthe film-mulched treatment. The N2O emissions persistently increased with an increasing Nrate, but the grain yield peaked in the treatment in which the N input (250kg N ha-1) wasnearly equivalent to the maize N uptake. Consequently, high yield (15.2-15.7t ha-1) and lowyield-scaled N2O emissions (125-155g N2O-N t-1grain) were simultaneously obtained in thetreatment with N fertilizer rate of250kg ha-1.