Mechanism Of Straw Returning On Nitrogen Transformation In Soil

Nitrate pollution in soil caused by excessive nitrogen fertilizer in agriculture is a common problem.It is one of the reasons of excessive nitrate in groundwater,which poses a potential threat to human and environmental health.Maize straw riches in organic matter,and returning to the field can not only improve soil physical and chemical properties,but also increase microbial activity and biomass.So maize straw is widely used as a soil amendment.In order to reduce nitrogen leaching and to explore the mechanism,maize straw was used as the research object,and evaluated the denitrification performance of the maize straw-soil system through batch experiments.Then built indoor soil columns carried out leaching experiments to investigate the influence of different conditions on soil nitrogen leaching and the transformation law of nitrogen fertilizer in soil.Finally,combined with molecular biology methods to clarify the influence mechanism and action mechanism of maize straw on soil nitrogen leaching.The main conclusions obtained are as follows:Maize straw is a slow-release natural organic material.The denitrification system using maize straw as a solid-phase carbon source and cropland soil as a bacterial source could achieve high NO₃^--N removal ratio（96.8%）within 120 h and no secondary pollution,with good denitrification performance.In the small-scale soil column leaching experiment,the soil column had the best control effect on NO₃^--N leaching with straw-to-soil mass ratio of 1%and nitrogen application rate of 100 mg·L^-1,and the NO₃^--N leaching ratio decreased to 0.9%gradually with straw decomposition.The detection results of TOC and DOM in the leachate showed that the addition of nitrogen was conducive to promoting straw decomposition,and the humic-like substances of DOM provided carbon sources for microbial activities.High-throughput sequencing analysis indicated that the reduction of NO₃^--N by soil column was the result of the combined action of anaerobic fermentation bacteria,organic matter degrading bacteria and denitrifying bacteria.At the same time,nitrogen application had a positive effect on increasing the abundance and diversity of microbial communities,and the amount of nitrogen application affected the distribution of microbial community structure such as Sporomusa.Furthermore,the results of leaching experiment sterilized soil showed that the microbial effect dominated the NO₃^--N inhibition process in this study,and the physical interception of soil could be ignored.Nitrogen transformation along the water flowing in the soil column showed that the mineralization of urea was realized through hydrolysis,nitrification and denitrification in the process of infiltration,in which the generated NH₄⁺accounted for a large proportion and could be adsorbed by the soil.The analysis of microbial community structure showed that the dominant genera in the upper soil layer（0～20 cm）of the soil column system were mainly related to nitrogen transformation,such as norank＿f＿＿norank＿o＿＿Vicinamibacterales.And the lower soil layer（20～38 cm）mainly enriched the genera related to anaerobic fermentation of straw,such as Hydrogenispora.In summary,straw returning is conducive to preventing soil nitrogen leaching,which provides sufficient carbon source for soil microbial proliferation and metabolism,and promotes the nitrogen transformation process in soil.