Belowground C sequestrations response to grazing exclusion in global grasslands: Dynamics and mechanisms

Globally, grazing exclusion is a widely implemented management strategy for restoring degraded grassland ecosystems and sequestering carbon (C). However, there is limited understanding regarding the temporal responses and underlying factors influencing ecosystem C stocks following grazing exclusion. In this study, we conducted a comprehensive synthesis of data from 199 independent experiments (454 pairwise observations) to analyze responses of plant and soil C stocks to grazing exclusion across four distinct grassland ecosystems (desert, typical, meadow, and alpine) in the globe. We found that rates of change in plant biomass C stocks and soil organic C stocks exponentially or rationally decreased with years since enclosure. Grazing exclusion generally enhanced aboveground biomass C in plants, while its effects on C stocks of belowground biomass and soil were more contingent upon various factors, such as climate, initial levels of C stocks, and grazing exclusion duration. Furthermore, the responses of C stocks of plant biomass and soil to livestock grazing cessation tend to stabilize over time, with equilibrium typically reaching after approximately 40 years, while soil C sequestration responses exhibited a lagged pattern compared to plant biomass C. Our results underscored the effectiveness of grazing exclusion as an effective strategy to enhance C stocks in regions characterized by low C content and non-water limited conditions. We propose that grazing exclusion for 1-5 years was the best restoration time for typical, meadow and alpine grasslands. Given the limited effects of grazing exclusion on soil organic C stocks of desert types, grazing exclusion might not be an effective measure to increase the soil organic C stocks in water-limited areas like desert grasslands.