Responses of plant productivity and soil nutrient concentrations to different alpine grassland degradation levels.

Although grassland degradation simultaneously affects plant productivity and soil nutrient concentrations, the relationship between plant productivity and soil nutrient concentrations during the process of grassland degradation is not yet well documented. A 4-year survey in the Qinghai-Tibetan Plateau was conducted to simultaneously investigate the relationships between plant productivity and soil nutrient concentrations in an alpine grassland at an overall degradation level and individual degradation levels. Our results showed that the total plant, sedge, and forb biomasses decreased, whereas the grass and legume biomasses first increased and then decreased as the level of alpine grassland degradation increased. Soil organic carbon (C), total nitrogen (N), total phosphorus (P), available N, and available P concentrations also decreased with the increase in degradation level. Our results also showed that plant productivity was positively correlated with soil nutrient concentrations (soil organic C, total T, total P, available N, available P) at an overall degradation level, whereas plant productivity was positively correlated with only the soil organic C concentration at each degradation level. Our findings suggested that the alpine grassland degradation conditions had different effects on the plant productivity of four functional groups (sedges, grasses, legumes, forbs) and affected the relationship between plant productivity and soil nutrient concentrations.