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.

Keywords: Grassland; Grazing; Meta-analysis; Methane uptake; Nitrous oxide; Soil respiration.