Coastal terrestrial-aquatic interfaces (TAIs) are dynamic zones of biogeochemical cycling influenced by salinity gradients. However, there is significant heterogeneity in salinity influences on TAI soil biogeochemical function. This heterogeneity is perhaps related to unrecognized mechanisms associated with carbon (C) chemistry and microbial communities. To investigate this potential, we evaluated hypotheses associated with salinity-associated shifts in organic C thermodynamics; biochemical transformations; and nitrogen-, phosphorus-, and sulfur-containing heteroatom organic compounds in a first-order coastal watershed on the Olympic Peninsula of Washington, USA. In contrast to our hypotheses, thermodynamic favorability of water-soluble organic compounds in shallow soils decreased with increasing salinity (43-867 ”S cm â1 ), as did the number of inferred biochemical transformations and total heteroatom content. These patterns indicate lower microbial activity at higher salinity that is potentially constrained by accumulation of less-favorable organic C. Furthermore, organic compounds appeared to be primarily marine-or algae-derived in forested floodplain soils with more lipid-like and protein-like compounds, relative to upland soils that had more lignin-, tannin-, and carbohydrate-like compounds. Based on a recent simulation-based study, we further hypothesized a relationship between C chemistry and the ecological assembly processes governing microbial community composition. Null