Grazing weakens N-addition effects on soil greenhouse gas emissions in a semi-arid grassland

Grazing and anthropogenic nitrogen (N) enrichment co-occur in most grassland ecosystems and may have substantial effects on production of soil greenhouse gases (GHGs). Although the individual effects of N addition and grazing on soil GHGs are well understood, their long-term interactive effects on grassland soil GHGs remain unclear. We conducted seven-year in situ measurement of three major GHGs in a long-term experiment comprising grazing (no, light, moderate, and heavy grazing intensity) and N-addition treatments (control, N addition: 10 g N m⁻² year⁻¹) in a semi-arid grassland, to determine the effects of N addition and grazing on GHGs. We found that moderate grazing reduced cumulative CO₂ emissions by 10%–11% compared with no, light, and heavy grazing. Unusually, CH₄ emissions from soils and N₂O uptake were found in this semi-arid grassland. Soil CH₄ uptake was markedly inhibited by moderate and heavy grazing. Relative to no grazing, grazing significantly reduced 60%–88% N₂O uptake over seven years on average. Nitrogen addition alone increased cumulative CO₂ emissions by 16% relative to control. An antagonistic effect between grazing and N addition was found on cumulative CO₂ emissions, cumulative CH₄ uptake, and global warming potential (GWP). Light grazing on this semi-arid grassland could offset 14% of the soil GHG emissions induced by N addition. Soil NO₃⁻ -N was the most important factor controlling soil CO₂ emissions and CH₄ uptake, and soil pH was a major factor mediating soil N₂O uptake or consumption. Our study highlights the importance that adjusting the grazing intensity of grassland is one of efficient strategies to mitigate GHGs emissions in the context of climate change.