Spatial source apportionment of n-alkanes and their effects on carbon pool dynamics and microbial communities in a...

The river-estuary-offshore continuum is a critical interface for carbon cycling. However, the regulation of its sedimentary carbon pools and microbial feedbacks by mixed sources remains poorly understood, particularly for small river systems. These globally prevalent systems are distinct biogeochemical hotspots, characterised by their tighter land-sea coupling. In this study, we investigated benthic sediments along the eastern Chinese coast to elucidate n-alkane sources, spatial distributions, and their relationships with carbon reservoirs and microbial assemblages, using gas chromatography equipped with a flame ionization detector (GC-FID) and high-throughput sequencing. Total n-alkane contents decreased markedly seaward, from an average of ∼4,705.7 ng/g dw in river sediments to 1,895.3 ng/g dw offshore, a trend primarily driven by terrestrial input dilution, further modulated by hydrodynamic sorting and degradation processes. Terrestrial plant waxes dominated biogenic inputs, as evidenced by abundant C₂₇-C₃₁ homologues and biogenic average chain lengths (ACL_bio) of 28.3-29.1, indicating mixed inputs from woody and herbaceous vegetation. Petroleum-derived anthropogenic inputs were relatively minor, consisting mainly of C₁₆-C₁₈ homologues, with anthropogenic chain lengths (ACL_anthro) of 20.3-23.5-suggesting levels below microbial degradation thresholds. Key mechanisms underpinning carbon pool stability included adsorption onto fine particles and alkaline sediment pH (8.2-8.7). Biogenic n-alkanes showed strong associations with sulphur-oxidising Campylobacterota (12.1% offshore), while lipid-adapted Proteobacteria (48.4% offshore) correlated with marine conditions, indicating preserved microbial diversity and functional redundancy. Despite low petroleum contents, offshore enrichment of Clostridia (14.2%) may reflect responses to trace hydrocarbons and/or low-oxygen conditions. We propose a conceptual "source-process-function" framework that links carbon source attribution, environmental filtering, and microbial response, offering an integrative view of carbon retention and transformation across dynamic land-sea transition zones, particularly in small estuarine systems.