Effects Of Elevated Atmospheric Ammonia Concentration On Root-shoot Characteristics,Nitrogen Utilization,and Yield Of Wheat And Maize Under Different Nitrogen Application Levels

The contradiction between the reduction of cultivated land area,the change of climate environment and the increasing demand for food by the population has become the key issue of agricultural development in the future.Due to the excessive application of nitrogen fertilizer by people,the concentration of ammonia（NH₃）in the atmosphere of farmland environment is constantly increasing,which has an impact on the root-shoot growth of wheat and maize,and the excessive nitrogen cannot contribute to the increase of crop yield,even will inhibit crop growth and cause the decrease of nitrogen use efficiency.How to control the application of nitrogen fertilizer to ensure the stability of crop yield under the background of levated atmospheric NH₃concentration has become the research focus of scholars at home and abroad.This experiment used an open-top air chamber and conducted a field microzone experiment using wheat（Xiaoyan 22）and corn（Zhengdan 958）as test materials.Two atmospheric NH₃concentrations(atmospheric NH₃concentration:0.01-0.03 mg·m^-3,AM;elevated atmospheric NH₃concentration:0.30-0.60 mg·m^-3,EAM)and three nitrogen application levels(0 kg·hm^-2,180 kg·hm^-2and 240 kg·hm^-2in wheat;0kg·hm^-2,225 kg·hm^-2and 300 kg·hm^-2in maize)were designed to study the effects of increasing atmospheric ammonia concentration on root-cap characteristics,nitrogen use and yield benefit of wheat and maize under different nitrogen application levels,in order to provide theoretical basis for nitrogen management in wheat and maize fields under increasing atmospheric NH₃concentration.The main conclusions of this experimental study are as follows:（1）Elevated atmospheric NH₃concentration and nitrogen application significantly increased the plant height,root index characteristics and biomass accumulation of wheat and maize.Under the elevated atmospheric NH₃concentration,the plant height,root length density,root surface area density and root volume density of wheat and maize at each growth stage under the conventional nitrogen application（N2）treatment were lower than those under the decreasing nitrogen application（N1）treatment.The average decreases of wheat were 9.82%,11.85%,7.42%,and 10.24%,and that of maize was 10.44%,22.03%,23.82%and 18.91%.Studies on the aboveground biomass and root biomass of wheat and maize during the harvest period found that there was also a downward trend,with the wheat was reduced by 13.72%and 19.35%on average.The maize was reduced by 7.52%and 11.26%on average.The results indicated that excessive nitrogen application was not conducive to the root crown growth of wheat and maize in the environment of increasing atmospheric NH₃concentration.（2）Elevated atmospheric NH₃concentration and nitrogen application significantly increased the changes in photosynthetic parameters,chlorophyll relative content and fluorescence parameters of wheat and maize,especially in wheat booting stage and maize tasseling stage.Under the elevated atmospheric NH₃concentration,the net photosynthetic rate,stomatal conductance,SPAD value and Fv/Fm value in all growth stages under conventional nitrogen（N2）treatment were decreased compared with those under reduced nitrogen（N1）treatment,among which the wheat was reduced by 14.00%,15.63%,7.37%and 3.71%on average.The maize was reduced by 14.71%,12.85%,7.36%and 4.01%on average.（3）Elevated atmospheric NH₃concentration and nitrogen application significantly increased the root-shoot nitrogen accumulation,nitrogen agronomic efficiency,nitrogen partial productivity and nitrogen use efficiency of wheat and maize.Under elevated atmospheric NH₃concentrations,grain nitrogen accumulation,above-ground nitrogen accumulation and root nitrogen accumulation under conventional nitrogen（N2）treatment were decreased compared with those under reduced nitrogen（N1）treatment,among which the average decrease of wheat was 15.50%,15.58%and 20.86%,and that of corn was23.73%,22.07%and 27.39%.Studies on nitrogen fertilizer agronomic efficiency,nitrogen fertilizer partial productivity,and nitrogen utilization efficiency of wheat and maize under different atmospheric NH₃concentrations showed that the indexes of conventional nitrogen（N2）treatment were significantly lower than those of reduced nitrogen（N1）treatment.Under the condition of increasing atmospheric NH₃concentration,the wheat was reduced by 86.40%,40.35%and 61.83%on average.The maize was reduced by 76.12%,42.08%and 73.83%on average.（4）The effects of atmospheric NH₃concentration,nitrogen application and their interactions on the number of grains per ear,grain weight,and grain yield of wheat and maize reached significant levels.The number of harvested ears,number of grains per ear,grain weight,grain yield,and net income of wheat and corn under different atmospheric NH₃concentrations showed the EAM treatment weas higher than the AM treatment.Under elevated atmospheric NH₃concentrations,the number of grains per ear,grain weight,grain yield,and net income of crops under conventional nitrogen（N2）application decreased compared to those under reduced nitrogen（N1）application,with wheat averaging 2.19%,8.20%,22.62%,and 23.44%,and corn averaging 10.36%,13.99%,21.96%,and 25.44%.Through path analysis of wheat and maize yields under different treatments,it was found that their aboveground biomass,aboveground nitrogen accumulation,root biomass,and root nitrogen accumulation were significantly positively correlated with the path analysis between the grain number and grain weight per ear of wheat and maize.These results indicated that photosynthetic indexes and root characteristic indexes of wheat and maize would indirectly affect grain number and grain weight per ear and yield through above-ground nitrogen accumulation and root nitrogen accumulation.In conclusion,it was determined that under the environment of increasing atmospheric NH₃concentration,root cap characteristics,photosynthetic physiology,nitrogen use and yield of wheat and maize in each growth period were the highest under the treatment of reduced nitrogen application（N1）.Therefore,we suggest that under the environment of increasing atmospheric NH₃concentration,the application of anthropogenic nitrogen fertilizer should be reduced,which is more conducive to nitrogen absorption and high and stable yield of wheat and maize.