Microbial nitrification and denitrification both can emit nitrous oxide (N2O), a major greenhouse gas, and the relative contribution of each pathway depends strongly on soil moisture conditions. We conducted a stable isotope tracer experiment to determine the contribution of nitrification and denitrification to N2O and dinitrogen (N2) fluxes in coastal plain wetlands, and to determine the response of these processes to changing soil moisture. We added 15N-labeled nitrate (NO3-) or ammonium (NH4+) to intact soil cores collected from an agricultural field, a restored wetland, and a preserved forested wetland, and subjected the cores to a drying or wetting hydrologic manipulation. Across all soils and treatments, the combined fluxes of N2O and N2 ranged widely, between 0.23 and 2900 μg N m-2 h-1, and N2O accounted for as little as 0% to as much as 43% of the total gaseous nitrogen (N) fluxes. Fluxes of both gases increased with increasing soil moisture in all soils and tracer treatments, but the relative enhancement of the two gases varied by soil type and N source. The N2O yields [N2O/(N2O + N2)] derived from both nitrification and denitrification were low (1-3%) in five of eight soils in each tracer experiment. Surprisingly, nitrification-derived N2O yields were highest (13-31%) in soils with the highest organic matter and soil moisture (restored wetland under simulated rain and forested wetland under drained and simulated rain), while denitrification-derived N2O yields (12-36%) were highest under simulated rain in the two mineral soils (agricultural field and mineral soils of the restored wetland), and under drained conditions in the forested wetland. These results are consistent with field-measured N2O fluxes in our previous work in these sites. We suggest that nitrification plays an important and underappreciated role in contributing to N2O fluxes from freshwater wetlands with often-saturated, acid-organic soils, while incomplete denitrification is the likely source of N2O following rain events in agricultural soils in southeastern U.S. coastal plain wetlands.
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