Climate trade-offs: Greenhouse gas emissions from natural and constructed intertidal wetlands.

Coastal wetlands have gained traction as Nature-Based Solutions, promising to mitigate climate change, improve water quality, and increase biodiversity. However, not every wetland can provide all these ecosystem services, and trade-offs must be recognised and managed. In this study, we tested whether improving water quality is at odds with climate change mitigation in a recently constructed coastal wetland designed as a nitrogen offset in subtropical Australia. We measured soil greenhouse gas fluxes (methane, CH4, carbon dioxide, CO2, and nitrous oxide, N2O) over two years, from the site's construction until it was connected to tidal waters. Simultaneously, we measured three reference mangrove and saltmarsh sites. Before construction, the site had low emissions (CO2 equivalents [CH4 + N2O] = 0.61 ± 0.49 kgCO2-eq ha-1 day-1) within the range of the natural reference sites (0.89 ± 0.52 kgCO2-eq ha-1 day-1). Emissions of N2O increased dramatically when the constructed wetland was fertilised (76.6 ± 72.1 kg CO2-eq ha-1 day-1) but decreased once the site was connected to tidal inundation and remained low for the next five months (0.34 ± 0.30 kgCO2eq ha-1 day-1). These results can serve as a first set of emission factors for constructed coastal wetlands designed as nitrogen offsets in Australia. In future projects, avoiding fertilisation, a practice that is not required for mangrove growth, will reduce emissions and increase the potential of this Nature-Based Solution to provide both water quality and climate benefits.