The pattern of soil microbe metabolic limitation was altered by the increased sheep grazing intensity in two contrasting grasslands: Implications for grassland management in semiarid regions

Soil extracellular enzyme stoichiometry (EES) and microbial metabolic limitation deeply reflect soil quality. However, knowledge about the impacts of different grazing intensities on microbial metabolic limitation has not been well-documented. Herein, the influences of four sheep grazing intensities (ungrazed, UG; lightly grazed, LG; moderately grazed, MG; and heavily grazed, HG) on microbial metabolic limitation were investigated in typical steppe (14-year grazing) and desert steppe (17-year grazing). The activities of an extracellular enzyme (EEAs) involved in soil C, N, and P transformation were determined by fluorimetric microplate assay. We found that different grazing intensities significantly affected extracellular enzyme activities and EES. Compared to the treatments of UG, LG, and MG, microbial relative C limitation in the HG treatment increased by 30.5-64.7% and 18.9-33.1%, respectively, in typical and desert steppes, indicating that heavily grazing aggravated soil microbial relative C limitation. Moreover, along with grazing intensity, vector angle decreased from 50.3 degrees to 27.6 degrees and from 71.5 degrees to 58.9 degrees in desert steppe and typical steppe. This indicated that the pattern of microbial metabolism limitation shifted from P limitation to N limitation in desert steppe, and heavily grazing significantly aggravated microbial N limitation in desert steppe. In addition, the desert steppe was more pronounced than the typical steppe to grazing intensity increasing. Therefore, heavily grazing aggravated microbial metabolism limitation, deteriorated grassland quality. The outcomes of this study highlight that lightly grazing can be an effective management practice to maintain grassland ecological services in semi-arid areas from the perspective of soil microbial metabolic limitation.