Gross N2O production drives net N2O fluxes along a salt marsh redox gradient

Background/Question/Methods

Future sea level rise will change the areal extent of salt marshes that are tidally inundated and the frequency of inundation, potentially altering redox dynamics that control nitrous oxide (N₂O) production and consumption via denitrification, a major nitrogen (N) loss pathway in coastal ecosystems. We conducted a field study to determine if salt marsh zones at different elevations (low, mid, and high marsh) experience different redox conditions that drive variations in N₂O dynamics. We established four replicate plots spaced 10 m apart along a transect in each of the marsh zones and sampled at low tide on four dates from December 2011 through June 2012. We used the ¹⁵N₂O pool dilution technique to measure field rates of gross N₂O production and consumption over three hour sampling periods. We also measured soil oxygen (O₂) concentrations in soil equilibration chambers buried at 10 cm depth in each plot. After each gas flux measurement, we collected a soil core from the chamber footprint and assayed it in the lab within four hours for ferrous iron (Fe(II)) and nitrate (NO₃^-) concentrations as additional indicators of soil redox conditions.

Results/Conclusions

Soil redox decreased along the marsh elevation gradient as evidenced by lower soil NO₃^- concentrations and higher Fe(II) concentrations in the low marsh compared to the mid and high marshes (p < 0.001 for both, n = 44). Soil O₂ concentrations were also lower in the low and mid marsh compared to the high marsh (p < 0.001). The low and high marshes were net N₂O sources, averaging 234 ± 129 µg N m^-2 d^-1 and 16 ± 13 µg N m^-2 d^-1, respectively. The mid marsh was a net N₂O sink, averaging -53 ± 23 µg N m^-2 d^-1. Both net N₂O fluxes and gross N₂O production rates were highest in the low marsh and lowest in the mid marsh (p = 0.009 and p < 0.001, respectively) whereas gross N₂O consumption did not differ among marsh zones. Thus, the variability in gross N₂O production drove the differences in net N₂O fluxes among marsh zones.  The non-monotonic trend in N₂O dynamics across the marsh elevation gradient suggests that the impact of future sea level rise on the net N₂O balance of this salt marsh will depend on the change in the areal extent of each marsh zone.