Effects Of Nitrogen And Phosphorus Addition On Soil Microbial Community And Carbon Mineralization In An Alpine Grassland

As the"decomposer"and"contributor"of soil organic carbon（SOC）,microbes directly involve in the mineralization and formation of SOC,which affect the accumulation and stability of SOC.Due to excessive utilization of fossil fuel and chemical fertilizer,nitrogen（N）deposition and phosphorus（P）inputs has become one of trends in global climate change.The Qinghai-Tibetan Plateau（QTP）is experiencing an unprecedented increase in N and P input.N and P regulates the growth of plants and microbes and microbial community structure and diversity,which further influences the formation and transformation of SOC.Hence,based on the long-term nutrient addition field experimental platform at the Haibei Alpine Grassland on the QTP,topsoil（0-10 cm）and subsoil（20-30 cm）collected at the second（2012;short-term nutrient addition）and tenth year（2020;long-term nutrient addition）were used to measure bacterial and fungal diversity and community composition with high-throughput sequencing method.Meanwhile,a soil incubation experiment with and without ¹³C-labeled glucose was conducted to examine mineralization potential of soil organic carbon(R_SOC).The study attempted to explore the response of microbial community diversity to short-and long-term nutrient addition.The dominant factor influencing R_SOC was identified with the analysis of soil physicochemical properties,plant biomass and microbial community properties.The main findings of the study are as follows:（1）Short-and long-term nutrient addition have different effects on bacterial and fungal richness and diversity.As fungi were more abundant in particulate organic matter（POM）with weaker protection and responded quickly to the environmental change,Short-term nutrient addition significantly changed fungal richness.In addition,as alpine grasslands on the Tibetan Plateau are more strongly in the state of P limitation,short-term P and N+P additions alleviated the P-limitation of fungi to some extent and increased their richness;Whereas bacteria were more abundant in mineral-associated organic matter（MAOM）which provided strong mineral protection,bacterial richness and diversity responded long-term nutrient addition significantly.Further,as the optimum p H range for bacteria was narrow and weakly alkaline,long-term N addition caused weak acidification of the soil by degrading soil p H,thus indirectly reducing bacterial richness and diversity in the topsoil.（2）Short-and long-term nutrient addition have different effects on R_SOC.Short-term N+P addition reduced R_SOC in the topsoil,indicating short-term N+P addition increased the contents of N and P in soils,weakened the nutrient limitation of microbes to some extent,inhibited the decomposition of SOC to acquisition energy,leading decreased R_SOC.The decreased R_SOC in the topsoil after long-term N addition suggested that long-term N addition enhanced fresh substrates from plants into soils by increasing plant biomass.Microbes preferred labile substrates as energy sources,when substrates were sufficient,which reduced the degradation of soil organic carbon,leading to decreased R_SOC.（3）After short-and long-term nutrient addition,soil physicochemical properties had a stronger effect on R_SOCcompared with microbial community properties.Whether short-or long-term nutrient addition,microbial community properties had weaker effects on R_SOC compared with soil physicochemical properties.After short-term nutrient addition,total nitrogen（TN）was the most important factor affecting R_SOC.Consistent with the"nitrogen mining hypothesis",increased N availability in soils removed the nutrient limitation of microbes to some extent,inhibited the process of degrading SOC by microbes to obtain nutrients,thus resulting in lower R_SOC.After long-term nutrient addition,SOC was the most important factor dominating R_SOC.In line with the principle of"preferential substrate utilization",long-term nutrient addition increased fresh inputs into soil by plants,microbes preferred decomposing labile SOC,and the recalcitrant SOC was accumulated in the soils,which led to reduced R_SOC.Overall,the study investigated the effects of short-and long-term nutrient addition on microbial community and R_SOC with high-throughput sequencing method and an incubation experiment adding ¹³C-labeled glucose.All above results provided new theoretical evidences for predicting changes in soil microbial communities,key process and regulated mechanisms of R_SOC in alpine grassland on the QTP under the background of increasing N and P inputs,which contributed to achieve the goal of"Double Carbon".