Abstract: Imbalance of soil elements is one of the major factors driving alpine grassland degradation. Clarifying the processes and mechanisms for the imbalance of soil elements is important for the restoration and sustainable use of degraded alpine grasslands. To achieve this, we analyzed basic soil physicochemical properties across a chronosequence: a 1-year enclosed site as short-term restoration stage, a 6-year enclosed site as medium restoration stage, and a 15-year enclosed plot as long-term restoration stage in typical alpine grasslands in the Qinghai-Tibet Plateau. Restoration of degraded grassland did not change soil pH, but increased soil waterholding capacity and electricity. Soil organic C and total N contents changed greatly across the restoration stage, while soil P content remains unchanged. With increasing restoration duration, C∶N, C∶P and N∶P ratios and available N (especially ammonium) in soil remarkably increased. The ammonium content at long-term restoration stage was 3.1 times higher than that after one-year restoration. The restoration of degraded grasslands greatly increased aboveground biomass, resulting in the increased soil C and N inputs through plant litter and rhizodeposition. Increased C and N inputs by plants triggered rapid C and N turnover in soil. Such a strong positive feedback help restore the degraded alpine grasslands. These findings indicate that imbalance of soil elements should be responsible for the degradation of alpine grasslands, providing an important basis for the restoration of degraded alpine grasslands and sustainable use of alpine grasslands.

Key words: organic carbon, nitrogen and phosphorus nutrients, alpine grassland, degradation, restoration, nutrient imbalance.