Long-term active restoration of extremely degraded alpine grassland accelerated turnover and increased stability of soil carbon

Abstract

Soil nutrient contents and organic carbon (C) stability are key indicators for restoration of degraded grassland. However, the effects of long-term active restoration of extremely degraded grassland on soil parameters have been equivocal. The aims of this study were to evaluate the impact of active restoration of degraded alpine grassland on: (a) soil organic matter (SOM) mineralization; and (b) the importance of biotic factors for temperature sensitivity (Q₁₀ ) of SOM mineralization. Soils were sampled from intact, degraded and restored alpine grasslands at altitudes ranging between 3,900 and 4,200 m on the Tibetan Plateau. The samples were incubated at 5, 15 and 25°C, and Q₁₀ values of SOM mineralization were determined. Structural equation modeling was used to evaluate the importance of vegetation, soil physico-chemical properties and microbial parameters for Q₁₀ regulation. The Q₁₀ of N mineralization was similar among intact, degraded and restored soils (0.84-1.24) and was higher in topsoil (1.09) than in subsoil (0.92). The best predictive factor of CO₂ -Q₁₀ for intact grassland was microbial biomass, for degraded grassland was basal microbial respiration, and for restored grassland was soil bulk density. Restoration by planting vegetation decreased the Q₁₀ of SOM mineralization as soil bulk density, the most important negative predictor, increased in restored grassland. The Q₁₀ of SOM mineralization in topsoil was 14% higher than in subsoil because of higher microbial abundance and exo-enzyme activities. The NH₄ ⁺ content was greatest in intact soil, while NO₃ ^- content was greatest in degraded soil. The SOM mineralization rate decreased with grassland degradation and increased after long-term (>10 years) restoration. In conclusion, extremely degraded grassland needs proper long-term management in active restoration projects, especially for improvement of soil nutrients in a harsh environment.

Keywords: Tibetan grassland; active restoration; soil organic matter mineralization; soil warming; structure equation model; topsoil and subsoil.