Effects Of Crop Residue Return And Fertilizer N Reduction On Soil Fixed Ammonium

In farmland ecosystems,fixed ammonium acts as a temporary reservoir of soil nitrogen and plays an important role in crop growth.However,in the case of reducing fertilizer N input,how the fixed ammonium pool responds and the crop residue return to the field affects the change of soil fixed ammonium remains to be further clarified.Relying on the long-term positioning experiment,this study set up 0（S0）,33%（S33）,100%（S100）three crop residue return treatments and 240 kg N hm^-2（N240）,190 kg N hm^-2（N190）,135 kg N hm^-2（N135）,and 0（N0）4 fertilizer N reduction treatments.After 3 years of treatment,the effects of different crop residue return amounts and fertilizer N reduction applications on the total amount of soil fixed ammonium in 2018 and 2019 were studied.In 2019,the effect of different amounts of crop residue return to the field and the reduction of fertilizer N application on the total amount of fixed ammonium in the soil showed that the fertilizer N reduction in 2018 had no significant effect on the total amount of fixed ammonium in the soil.But the N0 treatment in 2019 was significantly lower than the fertilization treatment.The total amount of fixed ammonium in 2018 and 2019 was 444 and 378 kg hm^-2 on average under the S100 treatment,which decreased with the reduction of the crop residue return amount,and the average decreased by 6.9%and 6.3%,at the S0 treatment,reaching significance difference.Compared with conventional fertilization and full straw returning（S100N240）treatments,when the fertilization rate was 190 kg hm^-2,the reduction of fixed ammonium was higher than that of deficiency N,and the reduction of fixed ammonium was higher with the reduction of nitrogen application.The reduction amount increased gradually,but the reduction amount of fixed ammonium（61 and 51 kg hm-2）under S0N0 treatment was lower than that of nitrogen deficiency（80 and 79 kg hm-2）;crop residue return to the field can alleviate the reduction of fixed ammonium.During the corn harvest period,there was no significant change in soil NH₄⁺-N,and crop residue return to the field could effectively alleviate the soil loss through nitrification.During the maize jointing period,the amount of fixed ammonium was related to the return of straw to the field and the reduction of N application.The average amount of fixed ammonium decreased significantly at S0 by 2.9%as the amount of crop residue return was 385 kg hm^-2;under conventional fertilization conditions,the amount of fixed ammonium was 393 kg hm^-2,which was fixed at N0.The amount of fixed ammonium was significantly reduced by 3.5%;during the corn filling period,the amount of fixed ammonium was related to the return of straw to the field and the reduction of fertilizer N application.The amount of fixed ammonium decreased significantly at S0 by 3.8%on average;under conventional fertilization conditions,the amount of fixed ammonium was 405 kg hm^-2,and at N0,the amount of fixed ammonium was significantly reduced by 4.0%;Under the application conditions,fixed ammonium can compensate for the N demand of crops through desorption/release,but in the case of extreme nitrogen deficiency,the release of fixed ammonium cannot meet the N demand of crops.The nature of the straw itself led to the reduction of the amount of fixed ammonium by returning the straw to the field.Returning straw to the field during the grain filling and harvesting periods is beneficial to alleviate the decrease of fixed ammonium and maintain the stability of the fixed ammonium pool.