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

Abstract:

The impact of grazing on CO₂ and CH₄ fluxes from subtropical pastures and thus on the climate system is uncertain, although these systems account for a substantial portion of global carbon storage. We investigated how cattle grazing affects net ecosystem CO₂ exchange (NEE) and CH₄ emissions in subtropical pastures using the eddy covariance technique over two complete wet-dry seasonal cycles. Grazing increased soil wetness but did not affect soil temperature. By removing aboveground biomass, grazing consistently decreased gross primary productivity (16% and 8 % in 2013-2014 and 2014-2015) and reduced ecosystem respiration (Re, 20% and 38% in 2013-2014 and 2014-2015). Lower Re in grazed (GP) than in ungrazed pasture (UP) was also explained by decreased soil and heterotrophic respiration and root biomass. Grazing increased the net CO₂ sink strength of the pasture (-86 ± 5 gC m^-2 yr^-1 in GP vs. -76 ± 6 gC m^-2 yr^-1 in UP in 2013-2014; -118 ± 9 gC m^-2 yr^-1 in GP vs. +142 ± 6 gC m^-2 yr^-1 UP in 2014-2015). Over both wet-dry seasonal cycles, both ecosystems were net sources of CH₄, and variations in fluxes without cattle present were driven by changes in soil wetness and temperature. The presence of cattle and greater soil moisture cased by the removal of aboveground biomass, caused greater total net ecosystem CH₄ emissions from GP than from UP (16% and 8 % in 2013-2014 and 2014-2015). Wetter soils under GP were responsible for 21-56% of the difference in net CH₄ emissions between pastures, suggesting that enhanced CH₄ production from wetter soils due to cattle presence can be a major contributor to annual CH₄ fluxes. Combining CO₂ and CH₄ to calculate a C budget revealed that grazing increased the net C sink strength of the pasture (-72 gC m^-2 yr^-1 in GP vs. -66 gC m^-2 yr^-1 in UP in 2013-2014; -114 gC m^-2 yr^-1 in GP vs. +144 gC m^-2 yr^-1 in UP in 2014-2015). Accounting for NEE and the radiative forcing of CH₄, grazing increased the net global warming potential (GWP) of the pasture in 2013-2014, but decreased it in 2014-2015 when GP was a net sink for greenhouse gases (GHGs) but UP was a net source. Our results highlight the importance of quantifying CO₂ fluxes in tandem with CH₄ fluxes in grazing lands, suggest that grazing profoundly alters net ecosystem GHG fluxes and GWP of subtropical pastures and indicate that grazing could enhance the C accumulated in pastures.