Wetness modulates the effects of grazing on net ecosystem productivity in global grasslands

Overgrazing has caused widespread global concern. However, how grazing intensity affects ecosystem carbon dioxide (CO2) fluxes with wetness fluctuations in grasslands at a global scale remains poorly understood. Here we measured ecosystem CO2 fluxes covering the 7th to 11th years of a continuous grazing experiment in a typical steppe, and conducted a meta-analysis of 585 observations by collecting relevant data in global grasslands. Our experimental results showed that mean gross primary productivity (GPP) and ecosystem respiration (ER) were decreased under heavy grazing (HG) compared with control without grazing (CK), whereas net ecosystem productivity (NEP) was not different between HG and CK in the typical steppe. The NEP was higher under light grazing in the wetter years, but it was not different between LG and CK under drier years in the typical steppe. In contrast, grazing decreased GPP, ER and NEP, respectively, in global grasslands. Both our field experiment and the meta-analysis revealed that the response of NEP to light grazing was linearly correlated with the wetness index, which may be mediated by greater plant relative growth rate in wetter years. Higher wetness and aboveground biomass (AGB) increased the response of NEP to grazing in global grasslands. However, heavy grazing reduced NEP and AGB even under higher wetness indices in global grasslands, resulting from a loss of their resilience in long-term heavy grazing. These findings indicate that light grazing appears to be a promising management to promote plant relative growth rate and CO2 flux sequestration. Furthermore, major drivers influencing ecosystem CO2 fluxes in response to grazing were aboveground biomass, grazing intensity and wetness index. Overall, this study provides a global perspective on the response of ecosystem CO2 fluxes to grazing intensity and improves our knowledge of the factors influencing the response of ecosystem CO2 fluxes to grazing intensity.