Effects Of Chinese Milk Vetch Application On The Dynamics And Migration Characteristics Of Soluble Organic Nitrogen And Bacterial Community In Paddy Soil

As a relatively active part of soil labile organic nitrogen pools,soil soluble organic nitrogen?SON?plays an important role in the absorption and utilization by plants and soil microorganisms,nitrogen mineralization and leaching as well as other aspects in soil ecosystems on account of strong mobility and nutrient availability.Soil microorganisms are important components in agricultural ecosystems,and play major part in humus formation,organic matter decomposition,and nutrient recycling as well,which can be affected by cropping patterns,fertilization measuresas and soil physicochemical properties.In this study,three treatments of chemical fertilization?CK?,low Chinese milk vetch application rate?L?and high Chinese milk vetch application rate?H?were conducted in grey paddy by a two-year continuous field experiment.The dynamics and profile variations of soil soluble organic nitrogen,the leaching characteristics of dissolved organic nitrogen?DON?and the dynamics and mechanism of soil bacterial community were investigated using total organic carbon analyzer with nitrogen detector,continuous flowinjection analyzer,high-throughput sequencing platform and real-time fluorescence quantitative PCR technology,respectively.Redundancy analysis and canonical correlation analysis were performed to reveal key factors affecting the soil SON and bacterial community in paddy soils.The objective of this study is to provide a scientific basis for the prevention of non-point source pollution of nitrogen and consummating nitrogen recycling theory.The results showed as follows:?1?The concentration of SON in the arable soil showed a dynamic trend of decreasing-increasing-decreasing-increasing.The application of Chinese milk vetch can significantly increase the SON concentration of arable soil in paddy soil.Druing the rice growth stages,the mean concentration of SON in L and H treatments were 18.82%and 36.90%higher than that in CK treatment.The grey slope correlation analysis showed that soil physicochemical properties,enzymatic activity and soil microorganism had positive effects on the dynamics of SON,and soil total nitrogen,protease and soil microbial quantity were major driving factors affecting the dynamics of SON.?2?The profile differentiation of SON concentration in paddy soil was significant differences in the sampling layers,ranking as SON_{0-20 cm}>SON_{20-40 cm} > SON_{40-60 cm},and the concentration of SON in arable soil were significantly higher than those in the depth of 20-40 cm and 40-60cm.Effects of different fertilization treatments were mainly existing in arable soil,which showed a grudal decrease in the deeper layers.Soil sampling times,soil depth and their interactions had significant effects on the concentration of SON.During the entire sampling period,the concentration of SON in different treatments showed a dynamic trend of decreasing-increasing-decreasing-increasing-decreasing in the arable soil,but it was decreasing-increasing-decreasing-increasing in the depth of20-40 cm and 40-60 cm.?3?During the entire sampling period,the concentration of DON in the floor layer showed the trends of decreasing-sharply increasing-rapidly decreasing-slowly decreasing-relatively steady.There was a certain difference in the DON concentration of floor layer in different fertilization treatments in the same sampling period.Furthermore,the DON concentration of the floor layer in rice transplantation stage,seeding stage and tillering stage was ranked as CK>L>H,while it was H>L>CK in heading and flowering stage,ripening stage,before the Chinese milk vetch application and 5 days after the Chinese milk vetch application.?4?The DON concentration of soil leachate from before Chinese milk vetch application to rice seeding stage was expressed as DON_0-20cm in heading and flowering stage to ripening stage,indicating DON concentration has a tendency to migrating down and accumulate in the lower layers.In addition,with the decomposition of Chinese milk vetch and the development and growth of rice,the depth and quantity of DON gradually increase.?5?Soil microbial quantity in different rice growth stages at the same treatments exhibit an order of transplantion stage>heading and flowering stage>ripening stage>before Chinese milk vetch application>tillering stage.Compared with chemical fertilization treatment,soil microbial quantity in Chinese milk vetch application treaments significantly increase,especially in treated with high Chinese milk vetch application rate.?6?There were no significant differences in the index of Chao 1 and Shannon during whole rice growth stages among treated with Chinese milk vetch application.The index of Chao1 showed a dynamic trend of first increasing and then decreasing druing the whole rice growth stage,and higher value of Chao1 index was observed in heading and flowering stage.The index of Shannon in different soil sampling times at the same fertilization treatments had no significant difference.?7?The dominant bacterial phyla in grey paddy soil were Chloroflexi,Proteobacteria,Acidobacteria and Nitrospirae identified in three treatments,with relative abundance of 58.09%-89.85%.The relative abundance of Proteobacteria increases,while the relative abundance of Acidobacteria and Chloroflexi decrease after Chinese milk vetch treatments.Comparing with CK,statistical analysis of metagenomic profiles showed that the significantly different bacterial genus number was 3-12 and 4-7 in the treatments of L and H in rice growth stages,respectively,with the highest in rice transplantation stage.The PERMANOVA result indicated that the sampling times accounted for44.25%variation of bacterial community structure,while the Chinese milk vetch application rate explained 7.95%variation of bacterial community structure.Redundancy analysis showed that soil organic matter,total nitrogen,ammonium,soluable organic carbon,urease and protease were the key factors determining the variation of bacterial community structure.Furthermore,correlation analysis showed that relative abundance of the 11 dominant bacterial phyla groups were significantly related to soil physicochemical properties and enzyme activity.