Grazing alters the net C sink strength and the net global warming potential of a subtropical pasture

Abstract:

Grazing profoundly affects climate by altering the exchange of greenhouse gases (GHG; CO₂ and CH₄) between terrestrial ecosystems and the atmosphere. Little is known about how this disturbance affects the GHG exchange from subtropical pastures although they account for a substantial portion of global grazing lands. Here, we investigated how cattle grazing affect net ecosystem CO₂ exchange (NEE) and CH₄ emissions in subtropical semi-native pasture using the eddy covariance technique. Soil moisture was greater under grazed than ungrazed pastures but soil temperature was similar between treatments. By removing aboveground biomass, grazing reduced gross primary productivity (GPP, 16%). While ungrazed pastures had higher GPP than grazed pastures, they also had higher ecosystem respiration (Re, 20%) along with higher heterotrophic respiration. As a result, annual sums of NEE were similar in grazed and ungrazed pastures and both systems were net sinks for CO₂ (-86 ± 5 gC m^-2 yr^-1 in grazed pasture, and -76 ± 6 gC m^-2 yr^-1 in ungrazed pasture). Including C removal by grazers in the C budget, grazing reduced the C sink strength (250%) and grazed pasture became a net source of C to the atmosphere. Increased soil wetness and CH₄ production from enteric ruminant fermentation enhanced net ecosystem CH₄ emissions (16%) in grazed than in ungrazed pastures. The net global warming potential (GWP) was higher (34%) in grazed than in ungrazed pastures, but both systems were net sources of GHGs when accounting for the radiative forcing of CH₄. Our results suggest that grazing reduces the net C sink strength and increases the net GWP of subtropical pastures. Improved understanding of how grazing affects ecosystem GHG fluxes is essential to predicting the role of pastures on the global C cycle.