Study On The Resistance Mechanism Of Groundwater Nitrate Pollution By Groundwater Depth And Nitrogen Application Rate In The Wheat-maize Rotation Farmland

Nitrogen leaching is common in the farmland of the North China Region,and groundwater nitrate is on the rise as a whole.The over-standard nitrate rate in shallow groundwater changes in different region,which seriously threaten the safety of underground drinking water.The vadose zone is an important barrier to protect groundwater from nitrate pollution,the denitrification potential of vadose zone can effectively prevent and reduce nitrate.Excess fertilization may lead to a large amount of nitrate accumulation in soil,which would enlarge the risk of groundwater contamination by nitrate leaching.The purpose of this paper is to explore the influence mechanism of groundwater depth and nitrogen application rate on crop-soil-groundwater system and nitrogen migration.Based on the large-scale lysimeter observation platform,groundwater depth（2,3,4 m）and nitrogen application rate（300 kg/hm²,240 kg/hm²）were set to monitor index related crop-soil-groundwater,and analyze the effects of groundwater depth and nitrogen application rate on crop grain filling and water and nitrogen use efficiency.The effects of groundwater depth and fertilizer application rate on soil physical and chemical properties,soil enzyme activities,soil denitrifying microbial abundance and community diversity were analyzed.A structural equation model was established to analyze the specific path that the influence of groundwater depth and nitrogen application rate on nitrate and water storage in soil profile.Stepwise regression analysis was used to analyze the limiting factors of denitrification intensity in vadose zone.The control mechanism of groundwater depth and nitrogen application rate on nitrate pollution in groundwater was explored in order to provide reference for the green and sustainable development of agriculture by reducing agricultural non-point source pollution.The main research results are as follows:（1）The summer maize yield was significantly affected by nitrogen application rate,groundwater depth and their interaction.Nitrogen application rate indirectly affected yield by influencing 100-kernels weight,kernel per spike and rows per ear.The groundwater depth indirectly affects yield by influencing the 100-kernels weight.The effect of nitrogen application rate on summer maize yield was greater than that of groundwater depth.The winter wheat yield was significantly affected by nitrogen application rate and groundwater depth,but their interaction had no significant effect.The groundwater depth affected winter wheat yield mainly through the indirect effect of influencing grain weight per plant,spike number,kernel per spike and its own direct positive effect,while nitrogen application rate affected winter wheat yield indirectly through influencing grain weight per plant and spike number.Due to the high water and nitrogen utilization efficiency of crops at the nitrogen application rate of N240,so it is feasible to reduce nitrogen by 20%in summer maize and winter wheat in the area of 2-4 m groundwater depth.（2）Groundwater depth,nitrogen application rate and their interaction jointly affect crop surface soil environmental factors.nirS denitrifying bacteria were predominant in the topsoil during summer maize season,while which was nirK denitrifying bacteria in the winter wheat season.Proteobacteria was the main denitrifying bacteria type of nirS and nirK in summer maize and winter wheat.N300 application rate increased the denitrifying bacteria community differences of among different groundwater depths of summer maize and winter wheat seasons.The community composition of nirS and nirK denitrifying bacteria was significantly affected by groundwater depth,nitrogen application rate and their interaction in summer maize and winter wheat seasons,respectively.The community structure of nirK denitrifying bacteria in summer maize was affected by groundwater depth,nitrogen application rate,NO₃^--N,EC,SOC and TN.The community structure of nirS denitrifying bacteria was influenced by groundwater depth,nitrogen application rate,NO₃^--N and EC.The community structure of nirK denitrifying bacteria in winter wheat season was mainly affected by groundwater depth,nitrogen application rate,p H and EC.The community structure of nirS denitrifying bacteria was influenced by groundwater depth,nitrogen application rate,NO₃^--N and EC.（3）Soil environmental factors in vadose zone were affected by groundwater depth,nitrogen application rate and their interaction.Organic matter,nitrate nitrogen,urease,nitrate reductase,nirS,nirK were significantly positively correlated with denitrification potential（DP）,EC was significantly negatively correlated with DP,and available phosphorus,available potassium were significantly positively correlated with DP.Stepwise regression analysis showed that EC,organic matter and nirK could explain 73.8%of the variation in DP,organic matter and nirK have a significant positive effect on DP,EC has a significant negative effect on DP.（4）Nitrogen application rate and groundwater depth affect soil water storage and nitrate storage in vadose zone.Nitrogen application rate of N240 increased the water content of soil profiles,and the improvement effect was mainly concentrated in the soil layer below 60 cm.The soil nitrate storage in winter wheat season was 76.05～130.11 kg/hm²higher than that of summer maize season.The increased nitrate storage in winter wheat season was mainly concentrated in 0～60 cm soil layer.GW2 treatment increased soil nitrate storage in 100～200 cm soil layer,and accelerated the downward migration of soil nitrate.The thick vadose zone increased the length of nitrate transport path,which can make full use of the denitrification capacity of the lower soil profile,prevent and eliminate the downward migration of nitrate accumulated in the soil,and effectively reduce the risk of groundwater nitrate pollution.In addition,the direct positive effect of nitrogen application rate on soil nitrate storage was the strongest.The soil nitrate storage was affected by soil depth mainly by regulating soil water storage which was the most effective factors on soil nitrate storage.（5）Groundwater depth and nitrogen application rate together regulate groundwater environment.The groundwater nitrate concentration was positively correlated with nitrogen application rate,organic matter,DP,NO₃^--N,catalase and crop yield,negatively correlated with groundwater depth,p H,available potassium,WUE and NPP,and positively correlated with total phosphorus and NH₄⁺-N.Nitrogen application rate has an indirectly effect on groundwater nitrate concentration mainly by influencing p H,NH₄⁺-N and total phosphorus.The depth of groundwater affects the nitrate concentration mainly through its direct negative effect and the indirect effect of total phosphorus and NH₄⁺-N.In addition,the effect of crop nitrogen partial productivity on groundwater nitrate concentration was negative.Groundwater depth has the strongest effect on nitrate concentration in groundwater.Thick vadose zone can prolong the time of nitrate transport to groundwater and increase the storage of nitrate.Therefore,when formulating fertilization strategy for regional agricultural green development,both fertilizer application rate and groundwater depth should be considered.Especially in shallow groundwater areas,reasonable fertilization and irrigation strategies should be formulated in combination with irrigation,precipitation,fertilization and other agronomic practices.We should improve the crop water and fertilizer utilization efficiency,reduce the deep transport and leaching of nitrate,and effectively prevent groundwater nitrate pollution eventually.