Effects Of Fertilization On Soil Microbial Community Composition And Structure And Carbon Mineralization In Alpine Meadows

Climate change and human activities affect grassland function as carbon pool in the Qinghai-Tibet Plateau.Microbes are the main contributors to soil carbon output.Changes in the composition and structure of microbial communities can affect the carbon output process and further affect carbon storage.In this study,we examined the soil microbial community composition and structure of alpine meadows after9-year continuous fertilization(CK:no fertilization,N:single nitrogen fertilization,P:single phosphorus fertilization,NP:nitrogen and phosphorus fertilization)in the Qinghai-Tibet Plateau.In order to explore the mechanisms underlying soil organic carbon(SOC)decomposition and how microbial community composition and structure changes affect SOC content decline in response to fertilization,soil carbon mineralization was studied by adding ¹³C labeled vanillin.The main findings were as follows:1.Microbial biomass carbon:microbial biomass nitrogen(MBC:MBN)varied from 4.1 to 5.8.Fertilization increased the ratio of MBC:MBN.2.Proteobacteria(26.4-29.1%),Actinobacteria(22.7-26.7%),and Acidobacteria(13.8-15.2%)were the main bacterial phyla.Ascomycetes(61.6-75.5%),Basidiomycetes(15.8-26.2%)and Zygomycetes(7.0-11.5%)were main fungal plyla in our study site.3.Fertilization significantly affected microbial community structure:the bacterial community structure of N,P was significantly different from CK(expect NP).Compared with the CK,the relative abundance of bacteria which prefer to utilize complex organic carbon increased significantly under N,P treatment,for example:Xanthomonadaceae,Caulobacteraceae,Oxalobacteraceae,Acidobacteriaceae,Acidimicrobiaceae.The fungal community structure was significantly different among fertilization treatments.Compared with CK,the relative abundance of Ascomycota reduced while Glomeromycota,Chytridiomycota increased significantly under fertilization treatment.4.The changes in microbial community structure were influenced by the aboveground biomass of plants and the physicochemical properties of soil.Total phosporus(TP),p H and soil nitrate nitrogen(NO₃^--N)were the main soil physicochemical properties which affected the structure of bacterial community;soil nitrate nitrogen(NH₄⁺-N),p H,and NO₃^--N were the main soil physicochemical properties which affected the structure of fungal community.5.The respiration rate and the cumulative carbon mineralization under N and P fertilization were significantly higher than CK,with the pattern of NP>N>P>CK without vanillin amendment and NP>P>N>CK with vanillin amendment.The patterns of the cumulative priming effect and the amount of vanillin mineralized were NP>P>N>CK and NP>N>P>CK,respectively.6.Bacteria and fungi together explained 91.7%and 96.6%of the variation in decomposition of complex organic carbon with and without vanillin amendment,respectively.Compared with the microbial contributions to the variation in carbon decomposition without vanillin amendment,fungal contribution to the variation in carbon decomposition with vanillin amendment increased by 12.3%while bacterial contribution decreased by 28.3%.These results led to the following conclusions:1.Long-term fertilization affected the microbial community structure by increasing above-ground biomass and changing soil physicochemical properties(SOC?TN?TP?NH₄⁺-N?NO₃^--N?p H)in the alpine meadow of the Qinghai-Tibet Plateau.2.Fertilization promoted the transformation of microbial community composition to that mainly decomposes complex organic carbon(i.e.,stimulating the relative abundance of fungi and bacteria that prefer complex organic matter),which accelerated the decomposition of complex organic carbon in soil,which would weaken SOC sequestration of Tibetan meadows.