Linking molecular properties of soil organic carbon to emergent ecosystem functions in a tidally influenced landscape of the Pacific Northwest

Coastal landscapes and their terrestrial-aquatic interface (TAI) will be increasingly exposed to short-term tidal inundation due to sea level rise and extreme weather events. These events can generate hot moments of biogeochemical activity and also alter ecosystem structure if occuring frequently. However, such responses of these vulnerable ecosystems to environmental perturbations are poorly understood, limiting our ability to evaluate the contribution of local processes on global scale carbon and nutrient budgets. Here, we evaluated whether and to what degree seawater inundation impacts biogeochemical responses in soils collected along a naturally variable salinity gradient in a first order tidal stream floodplain in the Pacific Northwest. A laboratory incubation experiment simulating episodic inundation was performed to detect the impacts on soil carbon chemistry. We characterized carbon before and after inundation using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS), metabolite signatures via Liquid Chromatography-Mass Spectrometry (LC-MS), and high-frequency carbon dioxide (CO₂) and methane (CH₄) gas fluxes from intact soil cores. Following three inundation events, we observed significant decreases in the thermodynamic favorability of the remaining organic compounds in soils with high natural salinity as compared to low salinity soils. Low salinity soils showed higher average flux compared to high salinity soils following periodic inundation events. Seawater inundation led to distinct metabolite features in low salinity soils, with surficial soil preferentially getting enriched in phenolic compounds. Biogeochemical transformations inferred from FTICR-MS data showed an increase in total transformations with increasing salinity for soil cores from naturally low salinity exposure sites, likely suggesting higher microbial activity. In conclusion, ecosystem responses in a tidal landscape frequently experiencing seawater inundation preferentially influences terrestrial soils to behave as a carbon source. This response is likely a function of historical salinity gradient-driven molecular-level organic carbon characteristics.