The influence of tidal inundation dynamics on below ground 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, the diverse nature of microbe-mediated carbon transformations, and the inherent complexity of soil environments makes it challenging to evaluate tidally-mediated carbon biogeochemistry. Here, a spatially resolved study was conducted to evaluate carbon compound signatures and soil microbial community composition and assembly processes along a tidally-influenced TAI across salinity and vegetation gradients of a coastal watershed in the Pacific Northwest. Organic carbon (OC) content and composition in depth-resolved soil samples 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. Additionally, community assembly process variables were evaluated from 16S rRNA amplicon sequencing data to understand linkage between ecological structuring of microbes and OC characteristics. This research is part of PNNL’s lab directed research development project “Predicting Ecosystem Resilience through Multiscale Integrative Science (PREMIS).