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 (NO)-O-2 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.