Heavy grazing reduces grassland soil greenhouse gas fluxes: A global meta-analysis

Abstract

Grazing degrades worldwide grasslands and possibly suppresses soil greenhouse gas (GHG: CO2, CH4 and N2O) fluxes. However, the global patterns of these three gas fluxes in response to grazing and the general mechanisms remain poorly understood. Here, we performed a meta-analysis of 63 independent grazing studies that measured soil GHG fluxes across global grasslands. Our results revealed that light and moderate grazing had no significant effect on soil CH4 uptake, N2O and CO2 emission, but heavy grazing consistently reduced them. The magnitudes of their responses to grazing were regulated by grazing duration and precipitation. In comparison with CO2 emission, soil CH4 uptake and N2O emission were reduced much more under heavier grazing, longer grazing duration or less precipitation. The decrease in soil CO2 emission was possibly caused by grazing-induced reduction in root biomass and soil moisture, while the decline in soil CH4 uptake and N2O emission was due to decreased soil moisture and substrate availability. Overall, this study provides the first large-scale evaluation on three main soil GHG fluxes in response to grazing, highlighting grazing inhibition of GHG emission but at the cost of plant productivity and soil fertility. We call for future efforts to identify an appropriate grazing intensity that is optimal to balance these complicated impacts.