Greenhouse gas fluxes and porewater geochemistry following short-term pulses of saltwater and Fe(III) in a subtropical tidal freshwater estuarine marsh

Estuarine tidal freshwater riparian marshes experience short-term saltwater intrusions (caused by typhoons and hurricanes) and Fe(III) inputs (associated with run-off from upstream basins during rain events). However, the effects of Fe(III) inputs and of simultaneous increases of saltwater and Fe(III) on greenhouse gas (GHG) fluxes and porewater geochemistry of estuarine tidal freshwater wetlands have not yet been studied. This study applied monthly in-situ experimental manipulations of saltwater and Fe(III) additions within a tidal freshwater wetland (the Min River estuary, southeast China) dominated by Sagittaria trifolia, over two years. We examined changes in porewater geochemistry (i.e., SO42-, Cl-, NH4+, and NO3-), ecosystem GHG (i.e., CH4, CO2, and N2O) fluxes, and soil denitrification rates. Short-term saltwater intrusions did not cause a large increase in soil salinity but did decrease CH4 fluxes. Fe(III) additions also greatly decreased CH4 fluxes, however, the combined addition of saltwater and Fe(III) did not have a synergistic effect on the reduction of CH4 emissions. Further, CO2 emissions did not significantly change in response to either the individual or the combined saltwater and Fe(III) additions. Most interestingly, we found that the combined addition of saltwater and Fe(III) led to an increase in N2O fluxes of an order of magnitude compared to the control. Based on the results of sustained-flux global warming potential (SGWP) and sustained-flux global cooling potential (SGCP) models, our findings suggest that short-term pulses of both individual saltwater and the combination of saltwater and Fe(III) caused by storm fides and soil erosion following a typhoon or hurricane event could reduce the CO2-equivalent flux from subtropical estuarine tidal freshwater wetlands, although the effects may be temporary.