Soil, Water, and Plants: Water Chemistry and Inundation Influence Biogeochemical Fluxes through Coastal Forests

Terrestrial-aquatic interfaces (TAI) are understudied transition zones between purely terrestrial and purely aquatic ecosystems. Although they comprise a small global footprint, coastal TAIs are important as they play a disproportionately large role in transforming carbon (C) as it is exchanged between land and sea. The quantity of CO2-C emitted from rivers alone is close to the quantity of C sequestered annually in the terrestrial biosphere; coastal waters have only begun to be similarly counted. Coastal TAIs are also subject to regular tidal inundations and vulnerable to both sea level rise (SLR) and extreme storm events. The transport and transformation of C and nutrients across the TAI is influenced by parallel gradients in other components of the ecosystem such as soil, microbial communities, geochemically coupled reactions, and vegetation. We hypothesize that changes to inundation and water chemistry patterns will significantly alter the coupling of biogeochemical cycles and fluxes of material across coastal TAIs.

This research uses models to identify key processes and inform experiments and measurements. Monitoring studies and experimental manipulations have been established at two first order coastal watersheds differing in tidal magnitude and SLR—the Smithsonian Environmental Research Watershed (SERC) in Chesapeake Bay (low tides, rapid SLR) and at Beaver Creek (BC) on the Pacific Northwest coast (large tides, slow SLR). We predicted an extreme tidal event and its impact to soil biogeochemistry, which was validated by field observation and directed soil sampling for lab incubations. To further advance these models, experiments are integrated at both sites to identify controls on key processes: Soil transplants between seawater-affected and purely terrestrial locations are in place to determine the role of slope position vs vegetation in mediating greenhouse gas (GHG) production. Seawater applications, coupled to real-time measurements of tree sapflow, GHG emission, and microbiology assess the impact of water chemistry on the ecosystem. Long data records (SERC) and regional surveys (BC) are used to derive a generalizable understanding of how water, soil, plants, and biogeochemistry buffer and/or suffer the perturbations of inundation and seawater intrusion.