Gross nitrous oxide production and consumption along a salt marsh redox gradient

Coastal wetlands denitrify nitrate (NO3-)-rich urban and agricultural runoff, and thus decrease anthropogenic nitrogen loading on downslope aquatic ecosystems. Elevation gradients in coastal wetlands likely create redox gradients that result in a range of denitrification dynamics. Our objective was to determine if this redox gradient could elucidate the controls on nitrous oxide (N2O) production and consumption in a salt marsh bordering Tomales Bay, CA. We installed soil equilibration chambers to measure soil oxygen (O2) at 10 cm depth along a transect in each of three marsh zones: high, mid, and low (n=4 per zone). We used the stable isotope trace gas pool dilution technique to measure gross rates of N2O production and consumption over three hour sampling periods at low tide when the surface soils were not saturated. Intact soil cores (0-10 cm depth) taken from the flux chamber footprints were extracted for ammonium, NO3-, and ferric and ferrous iron (Fe(III) and Fe(II)) concentrations as well as assayed for denitrifying enzyme activity (DEA). We sampled on four dates to characterize N2O dynamics across a range of environmental conditions. Bulk soil O2 concentrations in the soil equilibration chambers were higher in the high marsh than in the mid and low marshes (p<0.001, n=44). Soil NO3- concentrations were significantly lower and HCl-extractable Fe(II) concentrations were significantly higher in the low marsh compared to the high and mid marshes (NO3- p<0.001, Fe(II) p<0.001, n=44). Despite differences in redox among the marsh zones, neither gross rates of N2O production (Figure 1a) nor consumption (Figure 1b) varied significantly among the zones. DEA also did not differ among marsh zones, with averages ranging from 136 ± 30 ng-N g-1 h-1 in the mid marsh to 550 ± 121 ng-N g-1 h-1 in the low marsh. Overall, this salt marsh was neither an N2O source nor sink, with net N2O fluxes averaging 51 ± 40 μg-N m-2 d-1 across all marsh zones and sampling dates. However, net N2O fluxes were negative in 29 out of 44 measurements. Sub-atmospheric soil N2O concentrations at 10 cm depth together with the quantification of significant gross N2O consumption rates suggest that the net uptake of atmospheric N2O by the soil occurred in all marsh zones. Boxplots of (1) gross N2O production rates and (2) gross N2O consumption rates along a salt marsh elevation gradient. The y-axes are shown on log10 scale.