Methane emissions in grazing systems in grassland regions of China: A synthesis

The effects of grazing on methane (CH₄) budgets are important for understanding the balance of greenhouse gas emissions and removals in grassland ecosystems. However, the CH₄ budgets of grazing systems, that is simultaneously considering CH₄ uptake by grassland soils and emissions from ruminant enteric fermentation, livestock folds and animal feces, are poorly investigated, particularly for Chinese grasslands, and thus, remained unclear currently. Here, a synthesis of 43 individual studies was carried out to assess the grazing season/annual CH₄ budgets and their responses to grazing in grassland ecosystems of China. The results showed that heavy grazing (HG) significantly decreased, while light grazing (LG) and moderate grazing (MG) had no significant effects soil CH₄ uptake, as compared to un-grazing sites. Grazing has shifted Chinese grasslands from a sink to source for atmospheric CH₄, and the grazing season/annual CH₄ budgets increased with increasing grazing intensity, while the offset of CH₄ uptake by grassland soils to total CH₄ emissions from sheep, sheepfolds and feces were exponentially decreased with increasing grazing intensity. Moreover, the herbage biomass (HBM), organic matter intake (OMI) and live weight gain (LWG) were decreased while CH₄ emission intensities (i.e., CH₄ emission per HBM, OMI, and LWG) were linearly increased with increasing grazing intensity. Our results demonstrate that mediating grazing intensity, e.g., from HG to LG, could yield the optimal balance between maintaining productive grasslands and meanwhile mitigating CH₄ emissions. This study could help for building strategies with implications for grassland management in China with similar CH₄ emission problems.