| Summary: | Coastal wetlands provide several valuable services, such as carbon (C) storage and nitrogen (N) removal. Although wetlands serve as net C sinks, wetland soils release greenhouse gases (GHGs) including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Wetlands can buffer the influx of nitrate (NO3-) by transforming it into gaseous N (N2O, N2) through denitrification microbial pathway. Salinity is a regulator of soil biogeochemistry and long- (e.g. saltwater intrusion) and short-term (e.g. storm surges, river diversions) exposures may affect soil GHG production and denitrification. In this study, soil GHG production and denitrification enzyme activity (DEA) rates were examined over the course of a growing season (May, July, October) in soils from a freshwater, intermediate, brackish, and saline marsh. The response of GHG production and DEA rates were determined both under ambient and altered salinities (0, 10, 20, 30 psu). Soil CO2 and CH4 production rates decreased by 83% and >99%, respectively from the freshwater to saline marsh at ambient salinity. Soil N2O production rates did not vary across marshes, whereas, DEA was highest in May in the intermediate and brackish marshes. Short-term salinity exposure increased soil CO2 production in May and October, however, in July, soils displayed lower quality organic matter (high soil C:N), constraining respiration rates. Short-term salinity exposure decreased CH4 production, but increased N2O production in all months. Soil DEA displayed minor decreases with short-term salinity exposure. Soil GHG production in low salinity marshes (e.g. freshwater) had stronger responses to short-term salinity exposure than high salinity marshes (e.g. saline). Collectively, these results indicate that GHG and DEA rates do not always show the same responses to long-term salinity exposure, which results in shifts of vegetation structures, microbial communities, and soil properties compared to short-term salinity exposure. Sustained shifts to fresher conditions along salinity gradients may increase soil CO2 and CH4 production and short-term salinity exposure may increase CH4 production, but decrease soil CO2 and N2O production. Restoration activities (i.e. river diversions) that consider the interactive effect of salinity on C and N cycling can help reduce GHG footprint and increase nutrient buffering capacities of coastal wetlands.
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