Effects Of Nitrogen Application Rates And Types On Soil Physical And Chemical Properties And Bacterial Communities In High-yielding Paddy Fields

The nitrogen application rate,as well as the ratio of chemical/organic fertilizers,in the middle and lower reaches of the Yangtze River in China is generally high,while the utilization rate of nitrogen fertilizer is rather low.This often leads to deterioration of farmland ecosystems and soil pollution,which is not conducive to the sustainable development of modern agriculture.Meanwhile,a considerable portion of China's agricultural organic solid waste has not been fully utilized and causes serious environmental problems.Properly recycled,these resources could otherwise play an important role in the development of circular agriculture.Soil microbes play a key role in connecting soil and plants.Improving soil biological properties will benefit soil fertility and thus crop yield.To study soil microbial community in high-yield rice fields helps to elucidate the relationship among soil,crop and fertilizer and provide a theoretical basis for fertilization practices.Therefore,it is of great significance to focus on the application rate and the type of nitrogen fertilizers,in order to increase the utilization rate of nitrogen fertilizer,improve soil fertility and maintain the ecological balance in high yield rice cultivation.In this study,the tillage layer soil of the treatments was subdivided into three layers,to investigate the variation of soil nutrients and soil microbial community structure diversity,aiming at providing a reference for optimal fertilization practices.Three experiments were presented in this paper.(1)The effects of nitrogen fertilizer rates(0,270,300,360 kg·hm-2 chemical fertilizers used);(2)The effects of nitrogen fertilizer types(rapeseed cake or vermicompost accompanied with inorganic nitrogen fertilizers);and(3)Effects of straw return(CK,wheat straw return,rice straw return,wheat and rice straw return).Soil physical and chemical properties and soil bacterial community structure were studied in the above treatments for consecutive two years.Soil samples were collected during harvesting time in 2016 and 2017,with soil pH,EC,water content,alkali nitrogen,tal nitrogen,available phosphorus,total phosphorus,available potassium,and organic matter determined.High-throughput sequencing were conducted to study he effects of different fertilization measures on soil microbial community The main findings are as follows:Soil available nutrient content and soil microbial community structure were affected by nitrogen application rate.Compared with control,nitrogen fertilizer application could reduce the soil moisture content of rice paddy during harvesting stage.The nitrogen application rate changed the contents of soil effective nutrient contents,and the effects in different growth stages depended directly on nitrogen application rate and indirectly on the uptake of crop,whose growth conditions determined by nitrogen application rate and cultivation management measures.High-throughput sequencing results showed that the three dominant bacterial groups in high-yield paddy soils were Proteobacteria,Acidobacteria,and Chloroflexi.There was a significant correlation between soil water content,pH,EC,total nitrogen,total phosphorus,organic matter and microbial community structureNitrogen fertilizer application accompanied with organic fertilizer could increase soil nutrient content and soil microbial diversity,as compared with on inorganic fertilizers.At harvesting stage soil,the application of organic fertilizer significantly increased the contents of alkali nitrogen,total nitrogen,available phosphorus and available potassium in the soil.Straw returning affected soil available nitrogen content during rice harvesting stage.The effects of wheat straw returning on the soil available nitrogen continued until the rice harvest period.The combination of wheat and rice straw return promoted soil nitrogen transformation and availability of various soil nutrients.Two-season straw returning decreased the richness and diversity of microbial community in 0-5cm soil layer,while increased the microbial community richness and diversity in 10-15cm soil layer,as compared with the treatment without straw return.