Long-term grazing exacerbates soil microbial carbon and phosphorus limitations in the desert steppe of Inner Mongolia - A study based on enzyme kinetics

Soil enzyme activities can be used to characterize microbial metabolism given their importance as catalysts in various biochemical processes. However, how soil microbial metabolism in the desert steppe responds to long-term grazing remains poorly understood. Here, utilized data from a 19-year grazing experiment with four grazing intensities (no grazing [CK], light grazing [LG], moderate grazing [MG], and heavy grazing [HG]) in Inner Mongolia, we evaluated microbial metabolic limitations using soil enzymatic stoichiometry, and assessed the effects of these limitations on microbial carbon use efficiency (CUE) using a biogeochemical equilibrium model. Activity of C-, N-, and P-acquiring enzymes as well as soil hydrology, nutrient content, and microbial biomass were quantified to determine the drivers of microbial metabolic limitations. Long-term grazing reduced soil carbon (C), nitrogen, and phosphorus (P) acquisition enzyme activity. Microbial metabolism was restricted by C and P in the absence of grazing, and these nutrient limitations became more severe with increasing grazing intensity. Furthermore, structural equation modeling revealed that the microbial biomass, available nutrients, and total nutrient ratios affected the extent to which C was limiting, while only available nutrients affected the severity of P limitation. Meanwhile, microbial CUE decreased with grazing intensity and was regulated by microbial relative C limitation. Our findings emphasize that long term light grazing did not change microbial carbon use efficiency. Thus, it may be a proper utilization way for regulating soil nutrient cycles and facilitating the sustainable management of regional grassland.