Wetlands are the major natural source of the greenhouse gas methane (CH4) and are also potentially an important source of nitrous oxide (N2O), though there is considerable variability among wetland types with some of the greatest uncertainty in freshwater mineral-soil wetlands. In particular, trace gas emissions from seasonal wetlands have been very poorly studied. We measured fluxes of CH4, N2O, and CO2(carbon dioxide), soil nutrients, and net primary productivity over one year in natural, restored, and agricultural seasonal wetland prairies in the Willamette Valley, Oregon, USA. We found zero fluxes for CH4 and N2O, even during periods of extended waterlogging of the soil. To explore this lack of emissions, we performed a laboratory experiment to examine the controls over these gases. In a fully-factorial design, we amended anaerobic soils from all wetlands with nitrate, glucose, and NaOH (to neutralize pH) and measured production potentials of N2, N2O, CH4, and CO2. We found that denitrification and N2O production were co-limited by nitrate and carbon, with little difference between the three wetland types. This co-limitation suggests that low soil carbon availability will continue to constrain N2O emissions and denitrification in these systems even when receiving relatively high levels of nitrogen inputs. Contrary to the results for N2O, the amended wetland soils never produced significant amounts of CH4 under any treatment. We hypothesize that high concentrations of alternative electron acceptors exist in these soils so that methanogens are noncompetitive with other microbial groups. As a result, these wetlands do not appear to be a significant source or sink of soil carbon and thus have a near zero climate forcing effect. Future research should focus on determining if this is a generalizable result in other seasonal wetlands.
Keywords: Carbon cycling; Co-limitation; Denitrification; Methane; Mineralization; Nitrate; Nitrous oxide; Pacific Northwest; Restoration; Wet prairies.