Coastal flooding events and belowground transport tip the methane balance of a first-order watershed in the Pacific Northwest

Methane (CH₄) emissions from wetland soils to the atmosphere commonly decrease from net positive in freshwater environments to net negative in coastal environments, where sulfate reduction suppresses methanogenesis. However, belowground transport of water and dissolved CH₄ from one watershed compartment to another (e.g., soils-to-river, soils-to-vegetation, or river-to-landscape during floods) allows CH₄ produced in one compartment (e.g., anaerobic soils) to bypass oxidation in other compartments (e.g., shallow aerobic soils). These "short-circuits" can result in perceived hot spots/moments that have not been incorporated into watershed-scale conceptual models or budgets.

This study examines CH₄ fluxes along each watershed component and landscape position—soils, tree stems, and surface water at upland, freshwater wetland, floodplain, and river channel sites—across a first-order coastal watershed in the US Pacific Northwest, Beaver Creek (BC). Likewise, CH₄ concentrations and stable isotopic composition were measured in soil porewater, tree stems, and river water to evaluate physical exchange of gas/water between each watershed sub-unit and the extent of CH₄ oxidation in transit. We determined that spatially-scaled CH₄ uptake by the upland forest soils and trees was nearly balanced by positive emissions from the freshwater wetland and river channel. Minimal CH₄ was transported from the watershed's headwaters (i.e., the wetland) to the river. River CH₄ emissions were fueled by sediment production and lateral transport from the floodplain. Coastal floodplain soils had near zero CH₄ fluxes. However, flood events and floodplain trees (which bypass soil oxidation) contributed positive emissions that resulted in the watershed being a net CH₄ source to the atmosphere in 2018. Tidal inundation of the previously freshwater floodplain was restored in 2014, thus, continued research will reveal how the CH₄ balance shifts as floodplain salt-exposure continues to increase.