Responses of soil organic carbon and nutrient stocks to human-induced grassland degradation in a Tibetan alpine meadow

Alpine meadow degradation is a major environmental problem in the Qinghai-Tibetan Plateau of China. While it is expanding at a rapid pace, quantitative information on the effect of grassland degradation on soil properties remains largely unavailable. This study aimed to evaluate the effect of vegetation cover reduction on soil organic carbon (SOC) and nutrient stocks. A degraded alpine meadow with vegetation cover gradients of 90% ± 6.6% to 70% ± 8.3% and 45% ± 8.7% was selected, and soil samples were collected at two soil depths (0–20 and 20–40 cm depths). The reduction in vegetation cover decreased soil sand, SOC, total nitrogen (N), available phosphorus (P), inorganic N, microbial biomass carbon and N, soil moisture, and the ratio of SOC to available P, but increased soil pH, bulk density, the ratio of SOC to total N, and clay and silt contents. The decline in vegetation cover from 90% to 45% significantly affected SOC, total N, total P, and available P stocks at 0–40 cm soil depths (p < 0.05, respectively), with losses as high as 3.89 kg m−2 for SOC, 0.47 kg m−2 for total N, and 1.05 g m−2 for available P. These values corresponded to depletion rates at 28.7%, 39%, and 21%. The increases were as high as 39 g m−2 for total P and corresponded to an increase rate of 15%. The structural stability index had a linear decrease trend with decreasing vegetation cover. The index was >9% at the COV90 site and was close to 5% at the COV45 site for 0–20 cm soil depth. These results confirmed that vegetation cover reduction is a key factor that influences the soil nutrient stocks and structural stability of the alpine meadow. The losses in SOC and nutrients by vegetation cover reduction on the alpine meadow can have a remarkable influence on a large scale. Meanwhile, the alpine meadow may have huge carbon and N sequestration potentials duo to the potential increase of vegetation productivity in this area.