The impact of water management practices on subtropical pasture methane emissions and ecosystem service payments.

Pastures are an extensive land cover type; however, patterns in pasture greenhouse gas (GHG) exchange vary widely depending on climate and land management. Understanding this variation is important, as pastures may be a net GHG source or sink depending on these factors. We quantified carbon dioxide (CO2 ) and methane (CH4 ) fluxes from subtropical pastures in south Florida for three wet-dry seasonal cycles using eddy covariance, and estimated two annual budgets of CO2 , CH4 , and GHG equivalent emissions. We also estimated the impact of water retention practices on pasture GHG emissions and assessed the impact of these emissions on stakeholder payments for water retention services in a carbon market framework. The pastures were net CO2 sinks sequestering up to 163 ± 54 g CO2 -C·m-2 ·yr-1 (mean ± 95% CI), but were also strong CH4 sources emitting up to 23.5 ± 2.1 g CH4 -C·m-2 ·yr-1 . Accounting for the increased global warming potential of CH4 , the pastures were strong net GHG sources emitting up to 584 ± 78 g CO2 -C eq.·m-2 ·yr-1 , and all CO2 uptake was offset by wet season CH4 emissions from the flooded landscape. Our analysis suggests that CH4 emissions due to increased flooding from water management practices is a small component of the pasture GHG budget, and water retention likely contributes 2-11% of net pasture GHG emissions. These emissions could reduce water retention payments by up to ~12% if stakeholders were required to pay for current GHG emissions in a carbon market. It would require at least 93.7 kg CH4 -C emissions per acre-foot water storage (1 acre-foot = 1233.48 m3 ) for carbon market costs to exceed water retention payments, and this scenario is highly unlikely as we estimate current practices are responsible for 11.3 ± 7.2 kg CH4 -C emissions per acre-foot of water storage. Our results demonstrate that water retention practices aimed at reducing nutrient loading to the Everglades are likely only responsible for a minor increase in pasture GHG emissions and would have a small economic consequence in a carbon market.