Effects Of Groundwater Depth And Nitrogen Application On Soil-Crop Water And Nitrogen Transport And Utilization

Groundwater is an important water source for crop growth,which can regulate the process of crop water use,but unreasonable groundwater depth may limit the normal growth of crops.Nitrogen(N)application is an important measure to improve crop yield and agricultural efficiency,but a series of environmental problems caused by excessive N application become increasingly prominent.N application reduction and efficiency improvement has always been one of the important research contents of agricultural green development.The traditional combination of water and N has not only been extensively analyzed in research,but also fully applied in production practice.However,there are few reports on how to optimize N application and the combined effect of groundwater and N application in shallow groundwater deep area.Therefore,it is of great significance to explore the mechanism of groundwater depth and N application on crop growth,water and nitrogen utilization,nitrogen accumulation and transformation in winter wheat-soil system,and to optimize N application in shallow groundwater depth,agricultural efficient and sustainable development and ecological environment protection in agricultural areas.The study was based on micro-lysimeters,which was conducted at the Chinese Academy of Agricultural Sciences Field Scientific Observation Test Station of Agricultural Water and Environment in Xinxiang,Henan Province during 2020-2022.The experiment was conducted to systematically study the effects of four groundwater depth and four N application rates on growth,yield and quality,groundwater consumption and utilization,accumulation,transport and distribution of dry matter and N in key growth periods of winter wheat.The main research results are as follows:(1)N application and groundwater depth had significant water-nitrogen coupling effects on growth,matter accumulation and transport and yield formation of winter wheat.Too high and too low groundwater depth,insufficient and excessive N application were not beneficial to growth,matter accumulation and transport and yield formation of winter wheat.Groundwater depth and N application could increase the yield of winter wheat mainly by improving ear characters and increasing the number of effective ears.The optimal groundwater depth threshold existed for winter wheat growth index,yield,dry matter accumulation and transport.The optimal groundwater depth of 0-150 kg/hm~2 and 240-300kg/hm~2 were 1.2-1.5 m and 0.6-1.2 m,respectively.The groundwater depth threshold decreased with the increase of N application rate.Increased N application at depth of 0.6-0.9 m promoted the growth,above-ground matter accumulation and transport,and increased the yield of winter wheat.However,N application of 300 kg/hm~2 at the depth of 1.2 m decreased the dry matter accumulation and plant N at the anthesis stage,and N application of 150-240 kg/hm~2 increased the growth index,dry matter accumulation and transport and yield.(2)N application significantly affected water and nitrogen uptake and utilization of winter wheat,but it was affected by growth stage and groundwater depth.Adding N application under the 0.6 m depth significantly promoted the daily evapotranspiration(ETa)rate of groundwater during the jointing to the filling stage of winter wheat,and increased groundwater consumption,ETa and water use efficiency(WUE)during the whole growth stage.The WUE under 300 kg/hm~2 treatment was significantly 14.81%higher than that under 150-240 kg/hm~2 treatment.But the daily groundwater consumption amount,groundwater consumption,ETa and WUE in the whole growth period were better under the application of 150 kg/hm~2 nitrogen at the depth of 1.2 m.N uptake efficiency,partial productivity and N use efficiency of 150 kg/hm~2 at 0.6-1.5 m depth were the highest,which were 43.18%-79.69%higher than those of 240-300 kg/hm~2.(3)The increase of groundwater depth led to the increase of residual nitrate in 0-60 cm main root layer soil,and the high N application aggravated the increase of residual nitrate in 0-60 cm main root layer soil.The residual accumulation of nitrate in the main root layer(0-60 cm soil layer)under the depth of 1.2-1.5 m was 1.51-4.18 times that under the depth of 0.6-0.9 m.The nitrate accumulation in the main root layer of 240-300 kg/hm~2 was increased by 1.99-6.45 times compared with 150 kg/hm~2treatment.(4)The apparent N loss equation of winter wheat-vadose zone was established to quantify the apparent N loss and its difference under different groundwater depth and N application combination.The soil apparent N loss was aggravated by increased N application at the depth of 0.6-0.9 m,and the N application at 240-300 kg/hm~2 was 4.86-7.02 times higher than that at 150 kg/hm~2.But the apparent N loss occurred when N application was more than 240 kg/hm~2 at the depth of 1.2-1.5 m,especially the apparent N loss was the highest in the combination treatment of 1.5 m groundwater depth with 300kg/hm~2,which was 1.50-15.89 times that of other combined treatments.(5)Groundwater depth significantly affected the species diversity and relative abundance of microflora at the GS interface,but its effect on microflora richness was significantly affected by N application.The groundwater depth only affected a small number of bacteria with a small relative abundance,and the relative abundance of bacteria decreased with the increase of groundwater water depth.Under no N application,the relative abundance of 9 bacteria,including Acidobacteria,Vibrio sulfomyces,Myxococcus,Spirospira nitrifying and NB1-j at the depth of 0.6 m treatment was significantly higher than that at the depth of 1.5 m treatment.But only three types of bacteria with less than 5%relative abundance were significantly affected by groundwater depth under 150-300 kg/hm~2 N application.