Effects Of Nitrogen And Phosphorus Additions On Soil Microbial Community Composition And Topsoil Organic Carbon Storage In Songnen Grassland

Nitrogen（N）and phosphorus（P）are two essential elements for organisms,play an important role in regulating ecosystem structure and function,and maintaining ecosystem stability.The continuous increase of N deposition accelerates the phosphorus limitation in the terrestrial ecosystem and then affects the soil carbon sequestration potential by altering soil physicochemical properties,plant and microbial characteristics.Soil microbes are responsible for the decomposition of vegetation and animal and the nutrients transfer from soil to plant.Soil microbial communities play vital roles in the nutrient cycle.The topsoil carbon pool with a depth of 0～20 cm is in close contact with the atmospheric carbon pool and vegetation carbon pool.It is the key area for monitoring the dynamic of the soil carbon pool.The influence of N and P additions on soil microbial community and topsoil organic carbon storage in the temperate meadow in northeastern China is still unclear so far.A long-term field experiment of N and P additions was conducted in Songnen grassland northeastern China,the response of soil microbial community composition,topsoil organic carbon storage,and their correlation were studied.The main results are as follows:（1）N addition significantly increased soil total C and N contents,and increased soil C/P and N/P.P addition significantly increased soil P content but decreased soil C/P and N/P.N addition and P addition had no significant interaction on soil total C,N and P contents.N addition and P addition interactively influenced on soil C/P and soil N/P.No significant interactive effects between N and P additions on soil C/N were detected.N addition significantly reduced the contents of MBP and MBN.P addition significantly increased the contents of MBN and MBP.N addition and P addition interactively influenced on the contents of MBN,but had no significant interaction on the contents of MBC and MBP.N addition and N+P addition significantly increased MBC/MBN and MBC/MBP.P addition decreased MBC/MBN and MBC/MBP.N addition and P addition had no significant interaction on MBC/MBN,MBC/MBP,and MBN/MBP.（2）N addition and P addition interactively affected the PLFA content of soil fungi and bacteria.N addition had no effect on the composition and richness of soil bacterial community,but significantly altered the composition of soil fungal community and increased the species richness of fungi.P addition significantly altered the composition of soil fungi and bacterial communities,reduced the species richness of bacteria,and increased the species richness of fungi.Proteobacteria and Acinetobacter are the dominant species in the bacterial community,Ascomycetes are the dominant species in the fungal community.Soil available phosphorus content,soil total nitrogen content and soil water content are the main factors affecting the composition of soil fungal community,while soil available phosphorus content and soil water content are the main factors affecting the composition of soil bacterial community.（3）N addition had no significant effect on soil CO₂flux and the absorption of CH₄by soil.P addition significantly increased soil CO₂flux and the absorption of CH₄by soil.N and P additions interactively affected on the soil CO₂flux.N addition significantly increased the contents of POC and MAOC,while P addition had no impact on the contents of POC and MAOC.There was no significant interaction between N and P additions on the contents of soil POC and MAOC.Significant interactive effects of N and P additions on POC/SOC was detected.N and P additions did not interactively affect POC/MAOC.N addition and P addition had no interactive effects on the organic carbon storage of topsoil.（4）There was a significant negative correlation between soil fungi and soil organic carbon physical components,and there was no significant correlation between soil bacteria and soil organic carbon components.There was a significant positive correlation between soil fungi and topsoil organic carbon storage,but there was no significant correlation between soil bacteria and topsoil organic carbon storage.The results indicated that the soil fungal community may play a vital role in regulating topsoil organic carbon storage.In summary,the effect of N and P addition on soil microorganisms and topsoil organic carbon storage in Songnen grassland was mainly influenced by the changes of soil nutrient elements and their stoichiometric ratio,resulting in the change of soil microbial structure composition and diversity.The response of soil bacteria and fungi to N and P addition was inconsistent.Soil fungi and bacteria were more sensitive to P addition than N addition.Soil fungal community can better reflect the response of soil microbial community to N and P enrichment in a temperate meadow ecosystem and plays an important role in the regulation of topsoil organic carbon storage.The addition of P can alleviate the negative effects of N deposition on soil microorganisms,but the short-term P addition will accelerate the decomposition of soil organic matter,which might increase the instability of the soil carbon pool.The research results supply a new theoretical basis for how to improve grassland carbon sequestration potential under the background of global change and help to better predict the response of terrestrial ecosystem carbon cycle to environmental change.