| In the agricultural production of Inner Mongolia Hetao irrigation district,the application rate of nitrogen fertilizer has increased year by year,but the utilization rate of fertilizer is low,which causes great resource waste and environmental pressure.The application of nitrogen fertilizer not only affects the physical and chemical properties of the soil,but also affects the microbial environment.Soil microbes not only participate in the cyclical transformation of matter and energy in the soil,but also drive the transformation and migration of nitrogen.Seeking appropriate nitrogen fertilizer application can provide a theoretical basis for formulating reasonable nitrogen fertilizer management measures,reducing nitrogen loss,ensuring the good development of farmland microbial environment and reducing agricultural non-point source pollution.Therefore,this study focuses on how to optimize the application amount of nitrogen fertilizer and how microorganisms play a role in the process of nitrogen transformation.Therefore,in this study,in order to optimize the application of nitrogen fertilizer and how microbes play a role in nitrogen transformation,field experiments on nitrogen transformation and migration and soil microbial characteristics of intercropped wheat were carried out under different nitrogen application rates.Through experimental research,the mechanism of yield and water and fertilizer utilization,soil fertility response mechanism,nitrogen-transformation and transport mechanism in crop-soil,soil microbial quantity,microbial biomass and soil enzyme activity feedback mechanism and microbial-driven nitrogen transformation mechanism were revealed.Based on the mechanism revealing,the system dynamics Vensim model was used to simulate the transformation and migration of nitrogen in intercropped farmland under different nitrogen application rates,and the mutual feed model of soil microbial and nitrogen transformation was established.Finally,the threshold value of nitrogen application based on the characteristics of nitrogen transformation and transport and soil microbiology was proposed.The main research conclusions are as follows:(1)The yield and water use efficiency of intercropped wheat-con increased first and then decreased with the increase of nitrogen application rate,and reached the highest when the nitrogen application rate was 180 kg·hm-2and 270 kg·hm-2,respectively.The maximum increase was 37.76%,27.08%,33.90%,and 28.55%,respectively.All the indexes of nitrogen use efficiency decreased with the increase of nitrogen application rate,and reached the maximum when the nitrogen application rate was 90 kg hm'2 and 135 kg·hm-2,respectively,but the smallest decline when nitrogen application was 180 kg·hm-2 and 270 kg·hm-2,respectively.(2)The soil moisture content and conductivity of intercropped wheat-corn decreased first and then increased with the increase of nitrogen application rate.The contents of organic matter,alkali nitrogen and available phosphoras in the soil increased first and then decreased with the increase of nitrogen application rate.The soil water and salt content was the lowest and the nutrient content was the highest when the nitrogen application rate was 180 kg·hm-2 and 270 kg·hm-2,respectively.(3)With the increase of nitrogen application rate,the total nitrogen uptake of intercropped wheat-corn increased after harvesting;the residual nitrate nitrogen,ammonium nitrogen content and nitrate nitrogen leaching loss increased after harvest.The apparent nitrogen loss and nitrogen surplus showed a gradual increase,which peaked when the nitrogen application rates were 270 kg·hm-2 and 450 kg·hm-2,respectively.The model after the parameter calibration test constructed by Vensim was used to predict the content of nitrate nitrogen and ammonium nitrogen.The threshold of nitrogen application based on Vensim model was determined to be 175 kg·hm-2 and 231 kg·hm-2,respectively.(4)Within a certain range of nitrogen application(wheat 0-180 kg·hm-2,corn 0-270 kg·hm-2),soil microbial quantity,biomass and enzyme activity i1creased with the increase of nitrogen application When the nitrogen application exceeded this range,the amount of soil microbial microorganisms,biomass and enzyme activity decreased.Appropriate nitrogen application(180 kg·hm-2 for wheat and 270 kg·hm-2 for corn)provided sufficient nutrition and energy for the growth,reproduction and metabolism of microorganisms,was conducive to the increase of soil microbial quantity and biomass,and stimulated the elhancement of soil enzyme activity.(5)The smaller the microbial quantity and biomass,the lower the microbial enzyme activity has less negative effect on nitrogen conversion,indicating that the appropriate microbial quantity,biomass and enzyme activity can significantly accelerate the conversion of nitrogen.The empirical formula of microbial and nitrogen transformation and mutual feedback prediction established by wheat belt and con belt determined that the nitrogen application thresholds were 185.87 kg·hm-2 and 261.67 kg·hm-2,respectively.(6)The nitrogen application thresholds of wheat and com based on nitrogen transformation and migration and microbial characteristic parameters were respectively 175.00?185.87 kg·hm-2 and 231.00?261.67 kg·hm-2.Compared with the local customary N application,reduced by 17.39-22.23%(wheat)and 29.28?37.57%(corn),respectively. |
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