Fencing as an effective approach for restoration of alpine meadows: Evidence from nutrient limitation of soil microbes

The effects of restoration on plant communities and soil nutrients have been extensively studied but knowledge of the metabolic requirements of microbial communities is limited, especially in fragile alpine meadow ecosystems. Here, vegetation and soil from four meadows (grazed meadow, fenced meadow, fenced + reseeded meadow, and undegraded meadow) on the Tibetan Plateau were investigated. The nutrient requirements of microbes represented by stoichiometry of extracellular enzyme activities related to soil C, N, and P acquisition (β-1,4-glucosidase, BG; β-1,4-N-acetylglucosaminidase, NAG; leucine aminopeptidase, LAP; and alkaline phosphatase, AP) and their possible environmental drivers were determined. Our results showed that enzymatic C:N:P acquisition in all the meadows deviated from 1:1:1, suggesting that soil enzymatic activity stoichiometry in Tibetan alpine meadows is not homeostatic. Microbial communities in grazed meadows were co-limited by soil N and P levels, and this limitation was closely associated with the stoichiometry of soil nutrients. Activities of BG, NAG + LAP, AP, and their stoichiometry (C:N, C:P, and N:P) in fenced meadows were 38.2%, 32.9%, 51.2%, 16.8%, 19.5% and 2.3% higher than those in grazed meadows. These results indicate that fencing can relieve the N and P limitations for microbial communities in alpine meadows. Seeding the fenced meadow did not increase the soil nutrient content, microbial biomass, or enzyme activity compared with the fenced meadow, possibly owing to the low competition of the seeded species for resources. Changes in extracellular enzyme activity and stoichiometry were better explained by dissolved organic C and microbial biomass N than by plant or other soil properties. Our results demonstrate the effectiveness of fencing on the restoration of degraded alpine meadows from the perspective of alleviating microbial nutrient limitations.