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

Abstract

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.

Keywords: Coastal wetland; Greenhouse gases; Herbaceous vegetation; Inundation; Redox potential; Soil organic carbon; Woody vegetation.