Effects Of Soil Fertility And Nitrogen Rates On Nitrogen Use Efficiency And Nitrogen Cycling-related Soil Microorganisms In A Double-rice Cropping System

Improving rice yield level is the main way to solve the world’s food demand and is of great significance to ensure food security in China.Improving soil fertility and applying nitrogen fertilizer in appropriate amounts are the keys to high rice yields and high efficiency.In recent decades,the organic matter content of paddy soils in southern China has generally shown an increasing trend due to the increase in yield and the proportion of straw returned to the field.However,current studies have mainly focused on the effects of soil fertility on the yield and nitrogen use efficiency of double-rice,while the microbial mechanisms related to the regulation of nitrogen use efficiency of double-rice by different fertility and nitrogen application are not well understood.Therefore,in this study,four N application treatments(0,90,150,and 210 kg N·hm-2,denoted as N₀,N₉₀,N₁₅₀,and N₂₁₀,respectively)were set on the same low-fertility（F_L）,medium-fertility（F_M）,and high-fertility（F_H）soils of the parent material（red clay developed in the fourth century）to clarify the effects of soil fertility and N application The effects of soil fertility and N application on the yield and N use efficiency of double-season rice were clarified.At the same time,real-time fluorescence quantification was used to analyze the differences in the abundance of soil microbial functional genes related to the nitrogen cycle of double-season rice,in order to reveal the microbial mechanisms of soil fertility and nitrogen application that regulate soil nitrogen use efficiency,and to provide a scientific basis for coordinating the nitrogen use efficiency of red soil and improving the model of nitrogen cycle in farmland.The main research results are as follows.Both increased soil fertility and N application significantly increased early and late rice yields,and both had a significant reciprocal effect on early and late rice yields.Compared with the N₀ treatment,the yield increase of increased N fertilizer treatment on low-fertility soil of early rice ranged from 151.93%to 171.67%,but the yield increase on medium-fertility（102.29%～144.66%）and high-fertility soil（35.12%～60.47%）was lower than that on low-fertility soil.The yield trend of late rice was consistent with that of early rice.The N₂₁₀ and N₉₀ treatments were optimal on low-and high-fertility soils,respectively.And the increase in yield of both rice seasons was mainly attributed to the increase in effective panicle number.The total aboveground nitrogen uptake of early and late rice increased significantly with soil fertility and N application,and there was a significant reciprocal effect of soil fertility and N application on the total aboveground N uptake of early rice.Nitrogen use efficiency on low-fertility soils of early and late rice tended to increase with increasing N application,and tended to decrease on high-fertility.Increasing soil fertility and N application both significantly increased the abundance of soil AOA-amoA,AOB-amoA,narG（except for the period of late rice tillering fertilization）,nirK,nirS,nosZ,nrfA（except for the period of early rice tillering fertilization）and nifH genes.And the abundance of AOA-amoA gene was two orders of magnitude higher than that of AOB-amoA gene.and the abundance of nirK gene a is greater than that of nirS gene.Total nitrogen uptake was significantly or highly positively correlated with soil AOA-amoA,AOB-amoA（except for the period of early rice panicle fertilization）,narG（except for the period of early rice panicle fertilization）,nirK（except for the period of early rice basal fertilization）,nirS（except for the period of early rice basal fertilization）,nosZ,nrfA and nifH（except for the period of early rice basal fertilization and late rice tilling fertilization）gene abundance.Among them,soil AOA-amoA and narG gene abundance had the most significant effect on total aboveground N uptake.Nitrogen use efficiency was significantly and positively correlated with the abundance of narG genes during the tillering fertilization period in early and late rice.Soil AOA-amoA and AOB-amoA gene abundance had the largest positive correlation coefficients with soil ammonium nitrogen（r²=0.95）and microbial biomass nitrogen（r²=0.96）,respectively.Soil nitrite reductase and nitrate reductase activities were significantly and positively correlated with soil narG,nirK,nirS and nosZ gene abundance.Among them,soil nitrite reductase activity（r²=0.97）had the largest positive correlation coefficient with nrfA gene abundance.Soil microbial biomass nitrogen and protease activity had the most significant effect on nifH gene abundance.In conclusion,soil fertility and nitrogen application may affect the activity of AOA-amoA and narG microorganisms mainly through ammonium nitrogen concentration and nitrite reductase activity,thus regulating the total aboveground N uptake and nitrogen use efficiency of double-rice.