Effects Of Exogenous Nitrogen On Soil Gaseous Nitrogen Loss In Artificial Pastures In Karst Areas

In recent years,Guangxi has implemented large-scale ecological restoration projects,such as returning farmland to forage grassland,have been implemented in the Karst areas of southwest China,where the ecology is fragile,the study on the rule of soil gaseous nitrogen loss in the cultivated grassland of Karst is of great significance to the improvement of soil nitrogen fixation and productivity and the reduction of greenhouse gas emission.This study was based on the field of nitrogen input and carbon-nitrogen cycle in Huanjiang Karst ecosystem observation and research station of Chinese Academy of Sciences.There are two modes of exogenous nitrogen input in this study,one is for nitrogen-fixing plants,the other is for nitrogen fertilizer application.It was selected as the research object,and the control(CK),Amorpha fruticosa planting(AF),nitrogen fertilizer addition(NH₄NO₃:50 g N m^-2yr^-1)(N)and Amorpha fruticosa planting+nitrogen fertilizer addition(N+AF)were set up.The effects of exogenous nitrogen input on soil microbial community,key functional microorganisms of nitrogen transformation and gaseous nitrogen(N₂O and NH₃)emissions were studied by field in situ monitoring and laboratory analysis.The main result are as follows:(1)The N₂O emission flux from the artificial grassland in Karst was relatively low(1.69?g m^-2 h^-1),and the artificial grassland might be a sink of N₂O in December(N₂O emission flux=-2.64?g m^-2 h^-1).N input significantly increased N₂O accumulation,and the NH₃ accumulation was higher in December than June.The NH₃accumulation was affected by season,and the NH₃ accumulation was higher in June,nitrogen fertilizer application in December significantly increased NH₃ cumulative volatilization.In general,nitrogen fertilization contributed to the loss of gaseous nitrogen of artificial pastures in Karst.(2)The soil available nitrogen content of artificial pasture was significantly higher in June than December in Karst.N+AF treatment significantly increased the content of NH₄⁺-N,N+AF treatment significantly increased the content of NO₃^--N,and N+AF treatment significantly increased the content of NO₃^--N,the content of nitrogen increased with AF treatment,which indicated that the effect of exogenous nitrogen input on soil nutrients of artificial grassland was seasonal in Karst.(3)Soil microorganisms were mainly bacteria(54.5%?58.8%),followed by actinomycetes(15%?20%)and fewer fungi(5.5%?8.4%).Planting nitrogen-fixing plants could increase PLFAs of soil fungi,especially arbuscular mycorrhizal fungi(AMF)PLFAs.The responses of soil microbial community structure to nitrogen input showed seasonal variation,and was more sensitive in December than June.(4)The results showed that the abundance of nif H,AOA,AOB and nir K genes were significantly affected by exogenous N input in June.High available nitrogen concentration inhibited the growth of Azotobacter nif H and promoted the growth of AOB.The relative abundance of non-dominant genera was increased while the relative abundance of dominant genera was decreased with the increase of nitrogen input of AOB.High nitrogen input also inhibited nir K gene abundance of denitrifying bacteria,while high nitrogen input also inhibited nir K gene abundance of denitrifying bacteria,however,exogenous nitrogen input did not significantly change the abundance of nos Z gene in denitrifying bacteria.In summary,exogenous nitrogen input accelerated N₂O emission flux and NH₃volatilization rate.The mechanism is that exogenous nitrogen input first increases the content of available nitrogen in soil,and then changes the structure of soil microbial communities and the gene abundance and community structure of the key functional microorganisms in nitrogen transformation,in turn,the N₂O emission and NH₃volatilization were accelerated.