Tidal rewetting in salt marshes triggers pulses of nitrous oxide emissions but slows carbon dioxide emission

Pulsed emission of CO2 following rainfall in terrestrial ecosystems is well characterized, but coastal wetlands show the opposite trend for CO2, and less is known about the responses of other biogenic greenhouse gases like CH4 and N2O. Tidal inundation in coastal wetlands such as salt marshes is an additional form of rewetting with the potential to cause biological pulses that may not resemble rain-induced pulses. Accurately predicting the carbon and nitrogen cycling in salt marshes requires estimates of pulse responses to both rainfall and tidal inundation, in addition to baseline greenhouse gas flux rates. We measured CO2, CH4, and N2O fluxes from salt marsh sediment cores while we simulated rewetting by rainfall and/or tidal inundation, and we measured the total and potentially-active microbial communities at the end of the experiment. Rewetting from tidal inundation and from rainfall led to a short term (begun within 1 h, concluded within similar to 1 d) pulse of N2O, a sustained drop in CO2 emission, but no change in CH4. The N2O pulse peaked on average at 20x and 10x baseline flux rates after tidal inundation and rainfall respectively. Peak N2O fluxes far exceeded the highest emission observed in field measurements at these sites. By global warming potential, the pulse of N2O only slightly offset the drop in CO2 emission, therefore rewetting had a net cooling effect on marsh radiative forcing. Microbial communities were similar in rewetted and non-rewetted cores two days after rewetting, which reflects rapid recovery and/or resilience of the community to rewetting. We estimated the contribution of pulsed vs. baseline N2O emissions to annual totals by modelling rewetting events over 65-year tide gauge and precipitation records, and found that pulsed emissions constitute the majority of salt marsh N2O emissions, and could constitute the entirety in marshes with a net-sink baseline.