Gross nitrous oxide production drives net nitrous oxide fluxes across a salt marsh landscape

Sea level rise will change inundation regimes in salt marshes, altering redox dynamics that control nitrification - a potential source of the potent greenhouse gas, nitrous oxide (N2O) - and denitrification, a major nitrogen (N) loss pathway in coastal ecosystems and both a source and sink of N2O. Measurements of net N2O fluxes alone yield little insight into the different effects of redox conditions on N2O production and consumption. We used in situ measurements of gross N2O fluxes across a salt marsh elevation gradient to determine how soil N2O emissions in coastal ecosystems may respond to future sea level rise. Soil redox declined as marsh elevation decreased, with lower soil nitrate and higher ferrous iron in the low marsh compared to the mid and high marshes (P < 0.001 for both). In addition, soil oxygen concentrations were lower in the low and mid-marshes relative to the high marsh (P < 0.001). Net N2O fluxes differed significantly among marsh zones (P = 0.009), averaging 9.8 +/- 5.4 mu g N m(-2) h(-1), -2.2 +/- 0.9 mu g N m(-2) h(-1), and 0.67 +/- 0.57 mu g N m(-2) h(-1) in the low, mid, and high marshes, respectively. Both net N2O release and uptake were observed in the low and high marshes, but the mid-marsh was consistently a net N2O sink. Gross N2O production was highest in the low marsh and lowest in the mid-marsh (P = 0.02), whereas gross N2O consumption did not differ among marsh zones. Thus, variability in gross N2O production rates drove the differences in net N2O flux among marsh zones. Our results suggest that future studies should focus on elucidating controls on the processes producing, rather than consuming, N2O in salt marshes to improve our predictions of changes in net N2O fluxes caused by future sea level rise.