Study On Determination Method Of Reasonable Nitrogen Application Of Major Grain Crops In North China Plain Based On Agronomic Effect And Environmental Benefit

The North China Plain is an important grain-producing area in China.In order to obtain high yields as much as possible,excessive nitrogen(N)application is common,and environmental problems caused by surplus nitrogen emissions are becoming more and more serious,while determining optimal N application rate is one of the effective methods to balance yield and environment benefit after N application.But there is no uniform way to determine optimal N application.In this study,taking grain field as the research object,field monitoring,literature data review and simulation was used to,firstly,analyze the relationship between nitrogen application rate and different indicators based on the simultaneous nitrogen application gradient experiment,and seven optimal nitrogen rates and their differences were determined.Secondly,a comprehensive data review of yield and environmental effects with N application in the North China Plain was conducted,and the rational nitrogen application rate of wheat at the regional scale was discussed.Then,traditional method of determining optimal N rate based on agronomic effects was changed,and research scheme of identifying maximum allowable N rate based on environmental effect was proposed.The main findings were as follows:(1)Crop yield has been the most commonly used indicator for identifying an optimal N application rate.However,yield rapidly increased with increase in N application rate up to a specific rate(which was considered as the optimal N application rate)-after which,yield increase became negligible and even began to decrease.This relationship can be described using quadratic curve.Similarly,the effect of N application rate on traditional economic income,economic benefit considering environmental cost,N uptake by grain,N uptake by plant,NUE,and N balance all showed a similar type of relationship as yield.Therefore,seven optimal N application rates could be identified according to these indicators.All these indicators have their own theoretical justification,and different indicators should be considered under different situations,for instance,main grain-producing farms should focus on achieving the highest yield and suburban farms should concentrate on economic income,while croplands near water sources or ecological reserves should consider environmental benefit.(2)In the North China Plain,our literature review showed that the relationship between relative wheat yields and nitrogen application rates was significantly correlated with the following binomial expression equation:y =-0.0005x2 + 0.2476x + 66.95,R2 = 0.53).The maximum wheat yield was 7 203 kg ha-1 under a nitrogen application of 247.6 kg ha-1 according to this empirical equation.Our environment benefits analysis demonstrated that decreasing maximum wheat yield to 98%would entail decreases in the necessary nitrogen application rate,residual inorganic soil nitrogen(0-100 cm),nitrate leaching and ammonia volatilization of 25.25%,20.17%,27.89%,and 38.80%respectively,on a regional scale.Furthermore,a 2%decrease in maximum yield was not statistically significant within a yield range of 6 000-9 000 kg ha-1.This suggests that decreasing maximum yields to 98%should not lead to a measurable decline in yield.In addition,the results of our field studies and model simulation agreed that wheat yields showed temporal and spatial variation.Our field studies in six different sites indicated that,under a nitrogen application rate of 200 kg ha-1,the average yield of six sites was 7 234 kg ha-1 and 7347 kg ha-1 in 2012 and 2013,respectively,and the yield fluctuation rate was between-23.78%and 33.07%.According to the model simulation,at a given site,wheat yield could vary within a range of-57.07%to 34.73%between 2007 and 2021.This further indicates that decreasing nitrogen application rates by an amount corresponding to a 2%decrease in maximum yield would not result in risks to yield but could dramatically decrease environment risks.Thus,overall we found that the rate of nitrogen application corresponding to a 2%decrease in maximum yield(185 kg ha-1)was determined to be an appropriate regional nitrogen application threshold for the North China Plain.This rate corresponds with the recommendations of the Ministry of Agriculture.(3)N leaching was the main pathway for N loss in plain dryland farmland,in order to prevent nitrate N from exceeding the standard caused by nitrogen leaching to the maximum extent,groundwater quality standard was used to measure water quality leached out of crop root zone.Without first considering the quantity of output,as long as the nitrate N content leached out of root zone did not exceed the requirements of the Groundwater Quality Standard,the culprit of groundwater nitrate pollution cannot be attributed to N leaching in farmland.Taking spring maize as the research object,relying on the large-scale lysimeter in-situ monitoring combined with DNDC model simulation and artificial simulated rainfall,characteristic of water and N leaching could be clarified,and maximum nitrate leaching amount allowed in the farmland could be determined based on water leaching and Groundwater Quality Standard,then N application threshold was confirmed according to the nitrogen leaching failure curve.After that,key period of leaching,key impact factors,occurrence conditions and risk response strategy were analyzed.Divided application period and application method of N application threshold was ascertained.This research plan could effectively change the traditional method recommended N application mainly taking into agronomic effects,and provide a theoretical basis for China to measure the reduction of fertilizer from the environmental perspective.