Effects Of Straw Returning Combined With Nitrogen Fertilizer On Nitrification And Denitrification Of Rice Rhizosphere Soil

Soil nitrification and denitrification are important links of nitrogen cycle in rice ecosystem,and they are important ways to cause nitrogen loss.The rhizosphere of rice field is the place for rice soil microbial interaction,and it is also a hot area for studying soil organic matter and nutrient transformation.At present,the effect of straw returning combined with nitrogen fertilizer on nitrification and denitrification in rhizosphere soil is still unclear.In this study,the rice wheat rotation soil in Taizhou and Wuxi areas of Jiangsu Province was taken as the research object,and the pot experiment was carried out in rice greenhouse by rhizosphere bag method.The four treatments were studied under the four treatments,i.e.no straw and nitrogen fertilizer（CK）,wheat straw returning to field（SN0）,wheat straw return to field with reduced nitrogen fertilizer（SN1）and wheat straw return to field with constant nitrogen fertilizer（SN2）.The potential of soil nitrification and denitrification,soil microbial biomass,carbon and enzyme activity were measured in rice rhizosphere/bulk,and the abundance of functional microorganisms and the changes of bacterial community composition of soil nitrification and denitrification were determined.The main factors that drive soil nitrification and denitrification and the related microbial changes in rhizosphere/bulk are analyzed under the combination of straw returning to field and nitrogen fertilizer:（1）Straw returning combined with nitrogen fertilizer increased the contents of EC,SOC,DOC,NO₃^--N and NH₄⁺-N in rhizosphere soil,and decreased the p H of rhizosphere soil.The straw returning to the field and nitrogen fertilizer changed the soil physical and chemical properties,and then affected the potential of soilnitrification and denitrification.For Taizhou soil in Jiangsu Province,compared with SN0,the combined application of straw returning and nitrogen fertilizer（SN1 and SN2）significantly increased the nitrification potential of rhizosphere Soil by 4.93%-7.83%,while the denitrification potential decreased by 1.83%-1.76%.Different from Taizhou soil,the nitrification and denitrification potential of Wuxi soil increased by6.2%-34.16%under SN1 and SN2.（2）No matter the application of reduced nitrogen fertilizer or constant nitrogen fertilizer,straw returning combined with nitrogen fertilizer increased the content of soil microbial biomass carbon and microbial biomass nitrogen in rhizosphere,about8.56%-34.58%,and changed the ratio of soil microbial biomass carbon to nitrogen.In addition,the activities of non rhizosphere enzymes in the rhizosphere of the two rice fields were also significantly increased under the condition of straw returning combined with nitrogen application.Compared with SN0,the activities of urease,hydroxylamine reductase,nitrate reductase,nitrite reductase,protease anddehydrogenase in rhizosphere soil under SN1 and SN2 treatments were significantly increased by 0.67%-62.67%,and the activities in rhizosphere soil were higher than those in non rhizosphere soil.（3）The abundance of nitrification functional genes（AOA and AOB）and denitrification functional genes（nir K and nir S）was increased by straw returning combined with constant nitrogen application.Based on high-throughput sequencing technology,it was found that Actinobacteria,Alphaproteobacteria,Anaerolineae,Deltaproteobacreria and Gammaproteobacteria were the dominant bacteria at phylum level,among which anaerolineae had the highest relative abundance.Straw returning combined with nitrogen fertilizer significantly changed the composition and diversity of soil bacterial community.The results of principal component analysis showed that there were significant differences in soil microorganisms between the two regions,indicating that soil basic nutrients were the controlling factors of soil microbial community structure.The results showed that soil p H,EC,TOC,DOC,NH₄⁺and NO₃^-had significant effects on the variation of soil bacterial community.