Spatial gradients in soil-carbon character of a coastal forested floodplain are associated with abiotic features, but not microbial communities

Abstract. 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 heteroatom content in a first-order coastal watershed in the Olympic Peninsula of Washington state, USA. In contrast to our hypotheses, thermodynamic favorability of water soluble organic compounds in shallow soils decreased with increasing salinity, 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/algal-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 microbial community assembly processes and C chemistry. Null modelling revealed strong influences of dispersal limitation over microbial composition, which may be due to limited hydrologic connectivity within the clay-rich soils. Dispersal limitation indicated stochastically assembled communities, which was further reflected in the lack of an association between community assembly processes and C chemistry. This suggests a disconnect between microbial community composition and C biogeochemistry, thereby indicating that the salinity-associated gradient in C chemistry was driven by a combination of spatially-structured inputs and salinity-associated metabolic responses of microbial communities that were independent of community composition. We propose that impacts of salinity on coastal soil biogeochemistry need to be understood in the context of C chemistry, hydrologic/depositional dynamics, and microbial physiology, while microbial composition may have less influence.