Increased CO2 Efflux from Retreating Salt Marshes Occurs Before Active Erosion

Coastal wetlands are providing a variety of ecosystem services, among others valuable carbon stores, collectively referred to as blue carbon. The carbon sequestration potential is constrained by the difference between organic matter burial and its decomposition. Soil conditions are influencing organic matter decomposition rates and must be accounted for in blue carbon budgets. The differential soil to atmosphere CO₂ efflux between salt marsh sites experiencing differing geomorphic conditions (eroding vs. prograding) was measured in this study performed in Scotland, UK. Further, potential processes responsible for soil to atmospheric CO₂ flux were determined, including groundwater level, soil temperature and soil characteristics (i.e., grain size, carbon content and carbon stable isotopes). Eroding salt marsh sites had a 26.48% higher CO₂ efflux than expanding sites. Generalised linear mixed effects model (GLMM) and Linear mixed effects model (LMM) showed the relationship between CO₂ efflux and tidal cycle, erosion status, and the distance to the seaward vegetation edge. The efflux of CO₂ from the salt marsh is influenced by the underlying geomorphological conditions. These results highlight that salt marshes should be regard as heterogenous systems, especially considering analyses of future carbon storage budgets.