Effect of inundation on greenhouse gas emissions from temperate coastal wetland soils with different vegetation types in southern Australia

Predicted sea level fluctuations and sea level rise with climate change will lead to inundation of coastal and estuarine soils. Coastal wetlands usually contain large amounts of organic matter, which can be potential sources of greenhouse gas emissions (GHGs; CO2, CH4, N2O) during decomposition, but there are limited studies on the effects of sea level variation on GHGs in coastal wetlands. We measured the effect of brackish water inundation and wetting and drying cycles on GHG emissions from coastal wetland soil cores that supported four different vegetation types: Apium gravedens (AG), Leptospermum lanigerum (LL), Phragmites australis (PA) and Paspalum distichum (PD) from the estuarine floodplain of the Aire River in south-western Victoria, Australia. Intact soil cores were incubated under either dry, flooded, or a 14 day wet-dry cycle treatments for a total of 56 days at a constant temperature of 23 °C. CO2, CH4, and N2O fluxes were investigated in closed chambers and measured with gas chromatography. In the dry treatment, a positive correlation was found between soil organic carbon (SOC) and CO2 flux, and between SOC and CH4 flux. Higher SOC is indicative of higher amounts of soil organic matter (SOM) which acts as a source of substrate for microbes to produce CO2 or CH4 emissions under aerobic or anaerobic conditions. The NO2− and NO3− concentrations were positively correlated with N2O emissions in the wet-dry cycle treatment. NO2− and NO3− provide a supply of substrate for denitrification. The flooded treatment decreased cumulative CO2 emissions by 34%, 25% and 14% at the LL, PA, PD sites, respectively, and decreased cumulative N2O emissions by 42%, 39% and 43% at the AG, LL and PA sites, compared to the dry treatment. The wet-dry cycle treatment and dry treatment decreased cumulative CH4 emissions for all vegetation types compared to the flooded treatment. The redox potential (Eh) was negatively correlated with CH4 flux and positively correlated N2O flux at all sites. This study highlights the significance of sea level fluctuations when estimating GHG flux from coastal and estuarine floodplains which are highly vulnerable to inundation, and the role of SOC and mineral N as important drivers affecting GHG flux.