The millions of wetland basins in the approximately 750,000 km2 Prairie Pothole Region (PPR) of North America have the potential to impact carbon budgets on a continental scale. Methane (CH4) is an important greenhouse gas emitted by PPR wetlands, but controls on emissions are not well-defined. Individual wetlands within a complex can range from fresh to hypersaline as a result of solute inputs via groundwater and evapoconcentration. The major ions SO42-, HCO3-, Ca2+, and Mg2+ are elevated in local groundwater due to interaction with glacial till in which pyrite has been oxidized and the resulting acidity has been neutralized by carbonate dissolution. It is suspected that SO42- exerts a primary control on CH4 emissions as observed for marine systems, but this has not been systematically investigated in the PPR. We combined long-term research on solute dynamics and CH4 flux across the water-air interface at the Cottonwood Lake Study Area in North Dakota. Methane flux data have been collected from 12 wetland basins on a biweekly basis during growing seasons since 2009.
The wetlands at the 92 ha site had SO42- concentrations that ranged from <0.1 mM in groundwater recharge wetlands to almost 30 mM in some discharge wetlands. Median CH4 flux from central positions in ponded water from each wetland (0.1 to 34 mg CH4 m-2 hr-1) decreased exponentially with increasing ponded water SO42- concentrations. One wetland, which may have unique hydrology due to an underlying sand lens, was inconsistent with this trend. Its median CH4 flux was relatively high (similar to freshwater wetlands) despite a SO42- concentration of approximately 6 mM. In general, sulfate control on methane emissions from PPR wetlands appears to be similar to that for coastal marshes.