Carbon and nutrient mobilization across inundation gradients in an emerging freshwater delta

Emerging freshwater deltas are comprised of gradients of soil age and inundation frequency that influence biogeochemical processes. Differences in inundation patterns coupled with soil chemical properties may affect whether carbon and nutrients transported from inland watersheds are stored in delta soils or released into the atmosphere or open ocean. This study uses laboratory incubations to evaluate the responses of soil, porewater, and headspace parameters to different inundation patterns in soils collected from two elevation transects of an emerging deltaic island in the Wax Lake Delta in Louisiana, USA, an active freshwater delta forming along the Louisiana coast in response to water diversions. In general, soils from the older region of the island exhibited higher CO₂ release and solute mobilization relative to soils from the younger delta region. In addition, persistent inundation was associated with significantly lower solute (DOC, Fe, P) and CO₂ release than soil exposed to persistently drained conditions or periodic flood-drain cycles. Our results indicate that biogeochemical processes, such as organic matter decomposition and microbial Fe cycling, may be limited by inundated conditions in subtidal zones but vary in response to differences in soil chemical properties (e.g., C and N content) across supratidal and intertidal regimes. These results demonstrate that hydrogeomorphic development, in addition to inundation patterns, regulate biogeochemical processes that produce solutes and gases in coastal systems.