Grazing alters net ecosystem C fluxes and the global warming potential of a subtropical pasture

The impact of grazing on C fluxes from subtropical pastures and on the climate system is uncertain, although these systems account for a substantial portion of global C storage. We investigated how cattle grazing influences net ecosystem CO2 and CH4 exchange in temporarily flooded subtropical pastures using the eddy covariance technique. Measurements were made over several wet-dry seasonal cycles. Grazing increased soil wetness but did not affect soil temperature. By removing aboveground biomass, grazing decreased ecosystem respiration (Reco) and Gross Primary Productivity (GPP). As the decrease in Reco was larger than the reduction in GPP, grazing consistently increased the net CO2 sink strength of subtropical pastures (55, 219 and 187 more C m-2 in 2013, 2014 and 2015). Enteric ruminant fermentation and increased soil wetness due to grazers, increased total net ecosystem CH4 emissions in grazed relative to ungrazed pasture (27% - 80%). Unlike temperate, arid and semi-arid pastures where cattle are the major source of CH4, soil was a greater source of CH4 to the atmosphere than cattle in grazed subtropical pasture. Enhanced CH4 production from wetter soils caused by grazers was a major contributor of differences in CH4 emissions between grazed and ungrazed subtropical pastures. Although grazing increased total net ecosystem CH4 emissions, it decreased the global warming potential of subtropical pastures and, when differences could be detected, increased the net storage of C. Our results indicate that grazing of subtropical pastures can have a cooling effect on the climate.