Subsurface carbon signatures in a coastal watershed experiencing tidal inundation

The influence of tidal inundation dynamics on belowground carbon pools is poorly understood across coastal terrestrial-aquatic interface (TAI) ecosystems. The dynamic environmental conditions of tidally-influenced landscapes, the chemically complex nature of carbon compounds, and the diverse range of microbe-mediated carbon transformations, make it challenging to evaluate tidally-mediated carbon biogeochemistry. A spatially resolved study was conducted to evaluate carbon compound signatures along a tidally-influenced TAI across salinity and vegetation gradients of a coastal watershed. Organic carbon (OC) content and composition in depth-resolved soil samples and in pore-water samples collected at high and low tide throughout the study domain were investigated using ultra-high-resolution Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) in addition to bulk soil analyses. The FTICR-MS data was integrated with environmental variables (soil carbon, nitrogen, sulfur, and plant-available nitrogen, soil salinity and pH; dissolved organic carbon, salinity, dissolved gas concentrations, and colored dissolved organic matter) to reveal associations with OC composition. The surface samples were dominated by carbohydrates, lipids, and unsaturated hydrocarbons; deeper soils and an upland, non-tidal site had greater abundance of high molecular weight compounds (tannins, condensed hydrocarbons, and lignin). Soil salinity, carbon and nitrogen were positively correlated with low molecular weight compounds. Simple carbon compounds in pore-water (carbohydrates, amino-sugars, and proteins) were positively correlated with salinity and dissolved nitrous oxide gas, while greater abundance of less easily degraded compounds (tannin, lignin) was positively correlated with dissolved carbon dioxide and methane, suggesting that high salinity facilitates release of low-molecular weight compounds but may protect these compounds from oxidation by restricting microbial respiration. We hypothesize that salinity gradients lead to preferential storage and cycling of belowground carbon pool in tidally-influenced TAIs. The results will be used to derive a mechanistic understanding of carbon and nitrogen cycling and storage in tidally-influenced terrestrial corridors.