Soil Nutrients Directly Drive Soil Microbial Biomass and Carbon Metabolism in the Sanjiangyuan Alpine Grassland

This study aimed to identify the biomass and carbon metabolism characteristics of the Sanjiangyuan (SJY) soil microbial community; extract the key factors influencing the microbial characteristics; and provide some theoretical support for research into the function of grassland ecosystems in the Qinghai-Tibetan Plateau. We collected 334 soil samples from 17 sampling sites in August 2017. Soil physiochemical characteristics, microbial biomass carbon and nitrogen, and carbon metabolism features were determined. Correlation analysis and a piecewise structural equation model were used to identify the major parameters impacting the microbial features. There were considerable changes in soil microbial biomass-related indicators between areas and soil layers. Soil microbial biomass was high in the eastern and southern areas, and low in the northern and western regions, although the microbial quotient was the opposite. Soil microorganisms have distinct degrees of use of six types of carbon sources in different regions, but are more inclined to use carbohydrates, amino acids, and polymers. The structural equation model shows that, in addition to the greater contribution of SOC, the direct impact on MBN was also partially contributed by plant-derived carbon and phosphorus, while MBC was mainly affected by SOC. Plant-derived nutrients (carbon, nitrogen, and phosphorus) and soil total phosphorus primarily affected carbon sources in 0-10 cm, while SOC mainly affected carbon sources at 10-20 cm. Carbon metabolism characteristics were more stable than microbial biomass. Soil and plant nutrients contributed differently to the variation in soil microbial biomass and the ability of soil microorganisms to use carbon sources. Although the influence of plant-derived nutrients on the use of carbon sources was stronger than that on microbial biomass, soil-related variables (particularly SOC and total phosphorus) were still more important and direct drivers of soil microbial biomass and carbon metabolism than vegetation factors.