The impact of grazing on C fluxes from pastures in subtropical and tropical regions and on the environment is uncertain, although these systems account for a substantial portion of global C storage. We investigated how cattle grazing influences net ecosystem CO and CH exchange in subtropical pastures using the eddy covariance technique. Measurements were made over several wet-dry seasonal cycles in a grazed pasture, and in an adjacent pasture during the first three years of grazer exclusion. Grazing increased soil wetness but did not affect soil temperature. By removing aboveground biomass, grazing decreased ecosystem respiration (R ) and gross primary productivity (GPP). As the decrease in R was larger than the reduction in GPP, grazing consistently increased the net CO sink strength of subtropical pastures (55, 219 and 187 more C/m in 2013, 2014, and 2015). Enteric ruminant fermentation and increased soil wetness due to grazers, increased total net ecosystem CH emissions in grazed relative to ungrazed pasture (27-80%). Unlike temperate, arid, and semiarid pastures, where differences in CH emissions between grazed and ungrazed pastures are mainly driven by enteric ruminant fermentation, our results showed that the effect of grazing on soil CH emissions can be greater than CH produced by cattle. Thus, our results suggest that the interactions between grazers and soil hydrology affecting soil CH emissions play an important role in determining the environmental impacts of this management practice in a subtropical pasture. Although grazing increased total net ecosystem CH emissions and removed aboveground biomass, it increased the net storage of C and decreased the global warming potential associated with C fluxes of pasture by increasing its net CO sink strength.

Subtropical and tropical wetlands play a prominent role in the global carbon (C) cycle; yet factors that influence their C fluxes remain uncertain. We collected measurements from a temporarily flooded subtropical wetland over 3 years to investigate environmental drivers impacting CO2 and CH4 fluxes. The wetland was a sink of CO2 (−469 to −380 g C‐CO2 · m−2 · year−1) and a source of CH4 (25.1 to 32.1 g C‐CH4 · m−2 · year−1) to the atmosphere. Dry season CH4 emissions represented 41 to 49% of the annual budget, reflecting the importance of continuous CH4 flux measurements. Gross primary productivity (GPP) increased with temperature and radiation, and the influence of VPD on GPP varied with soil inundation. Higher water tables decreased Reco and increased GPP, and a higher GPP in turn lead to enhanced Reco likely through enhancements of GPP on autotrophic respiration. This suggests that the impact of the water table on Reco depends on the cancelling effects of hydrology and GPP. Emissions of CH4 increased with soil temperature, water table, and GPP until soils were inundated at which point temperature and GPP became the main drivers. Water table and temperature influenced GPP and CH4 fluxes, and increases in GPP directly enhanced CH4 emissions. In addition to impacting C fluxes directly through water table depth, hydrology also determined the hierarchy of the dominance of factors controlling C fluxes and their response. The positive climate forcing of subtropical wetlands may be dictated by plant‐mediated and climate interactions, with hydrological factors playing a major role in determining the greenhouse gas sink or source strength of subtropical wetlands.

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