Effects of grazing exclusion on carbon sequestration in China's grassland

Globally, grazing exclusion (GE) is an effective management practice to restore degraded grasslands and improve carbon (C) stock. However, the C dynamics in grasslands ecosystem with GE have not been well characterized. The results of 145 sites published in 118 recent literatures were synthesized to examine the dynamics of plant and soil C sequestrations in grassland ecosystem after GE, and with the recovery age > 27 years under the China's 'Returning Grazing Land to Grassland' Project. Results showed a positive impact of GE on vegetation and soil C stock at most sites. The mean rate of aboveground biomass carbon stock (AGBC) change was 10.64 g m(-2) yr(-1), and the mean rate of belowground biomass (0-30 cm) carbon stock (BGBC) change was 32.14 g m(-2) yr(-1) after GE. The mean rate of soil C stock change was 0.27, 0.23, 0.18, 0.09 Mg ha(-1) yr(-1) in 0-10 cm, 10-20 cm, 20-30 cm, and 30-100 cm (equivalent to 10 cm), respectively. And Grass-dominated grasslands present a higher C sequestration ability than forb-dominated grasslands. Soil C stock rates and vegetation biomass C changes showed an Exponential Decay trend since GE, and the AGBC changes reached a steady state (when the rate at the equilibrium point) first, followed by BGBC, and then soil C. The AGBC and BGBC both had opposite views on soil C changes in the top 30 cm soil layers. Soil N is a key factor in the regulation of soil C sequestration since long term GE ( > 20 years). The large scale of GE under 'Returning Grazing Land to Grassland' Project significantly increased grassland C stocks. Meanwhile, increased soil N supply to grasslands with GE at the latter recovery stage may enhance ecosystem C sequestration capacity.