Soil stoichiometry modulates effects of shrub encroachment on soil carbon concentration and stock in a subalpine grassland

There is little information available on the mediating effects of soil nutrient stoichiometry and enzyme stoichiometry on soil carbon (C) during shrub encroachment and their contributions to changes in soil C. Here, we characterized the concentration and stock of soil organic carbon (SOC), inorganic carbon (SIC) and total carbon (STC) along the shrub encroachment sequence (SES). We constructed linkages between soil C and SES with soil nutrient stoichiometric ratios and C-, nitrogen- and phosphorus-acquiring enzyme stoichiometry ratios using structural equation modeling (SEM), and disentangled the contributions of the soil nutrient stoichiometric ratios and enzyme stoichiometric ratios to shaping SOC and SIC using redundancy analysis (RDA) and SEM. Results revealed that the increases in STC stock derived from the increases in both the SOC stock and the SIC stock. Soil stoichiometric ratios played a mediating role in structuring soil C over SES, the mediating pattern depended on soil stoichiometry types (nutrient stoichiometry or enzyme stoichiometry) and soil C types (SOC, SIC or STC). Soil nutrient stoichiometric ratios contributed more than soil enzyme stoichiometric ratios to the variation in SOC and STC, while the contributions of these two types of soil stoichiometric ratios to the variation in SIC changed with soil stoichiometry types. Soil nutrient stoichiometry had positive or negative or threshold effects on soil C, but soil enzyme stoichiometry had monotonously increasing effects on soil C. This study showed that the soil stoichiometry had modulatory effects on soil C during shrub encroachment in the subalpine grassland, China.