Global Meta‐Analysis Reveals the Responses of Carbon Cycle to Grazing Exclusion in Grasslands

ABSTRACT Despite a large number of field manipulative grazing exclusion experiments having been conducted on global grassland ecosystems, the general patterns of how grazing exclusion affects the ecosystem carbon cycle remain unclear. Research on the impact of grazing exclusion on the carbon cycle in grassland ecosystems will help to more accurately assess the role and effectiveness of grazing exclusion. Here, we used a comprehensive meta‐analysis to examine the responses of 12 variables associated with carbon pools and fluxes to grazing exclusion based on 226 studies in global grasslands. Our results indicated that grazing exclusion significantly increased carbon pools and fluxes in grassland ecosystems, except for methane emission (CH 4 ) and net ecosystem carbon exchange (NEE). Specifically, the effect size of litter increased the most at +4.90, followed by aboveground biomass (AGB) at +3.93, dissolved organic carbon (DOC) at +2.58, soil organic carbon (SOC) at +2.24, belowground biomass (BGB) at +2.14, microbial biomass carbon (MBC) at +2.12, carbon dioxide emission (CO 2 ) at +1.78, ecosystem respiration (ER) at +1.17, soil respiration (Rs) at +0.93, and net primary productivity (NPP) at +0.72. Grazing exclusion duration and climate factors were the primary drivers of changes in carbon pools and fluxes. We demonstrated the time thresholds by establishing the general response curves of changes in carbon pools and fluxes with grazing exclusion duration. Moreover, AGB, BGB, SOC, DOC, Rs, NEE, and CO 2 showed an initial rise and then a decline with grazing exclusion duration, and there was a time threshold which was 17 years, 19 years, 26 years, 6 years, 7 years, 30 years, and 8 years, respectively. We found a significant decrease in DOC (slope: −0.59), litter (−0.28), CO 2 (−0.24), NPP (−0.17), and NEE (−0.04) with mean annual temperature. We also found a significant decrease in CH 4 (−0.008), NEE (−0.006), MBC (−0.003), ER (−0.003), and AGB (−0.002), but a significant increase in CO 2 (+0.011) and NPP (+0.009) with mean annual precipitation. Overall, when carbon sequestration is the goal of management, the role and application of grazing exclusion in grasslands should be reconsidered in terms of grazing exclusion duration and climate factors.