Persistent carbon composition converges across vegetated and unvegetated coastal soils despite contrasting...

Coastal soils store organic carbon across a range of timescales, yet whether the composition of the preserved pool reflects input pathways or post-depositional selection has remained unresolved. We characterized the molecular composition of soil organic carbon (SOC) in 50 samples collected from 25 pairs of vegetated and unvegetated coastal sites along a 20° latitudinal gradient of the Chinese coast, combining pyrolysis-GC/MS and THM-GC/MS fingerprinting with co-occurrence network visualization, multivariate ordination, and structural equation modeling. Plant-driven and mineral-driven stabilization dominated vegetated and unvegetated habitats, respectively. Mangroves stored 3.3 times more SOC than adjacent mudflats, whereas salt marshes showed only a 1.3-fold enrichment. Mudflats were enriched in marine-derived C relative to their vegetated counterparts. Despite these divergent inputs, the resistant fraction converged across all habitats on aliphatic and degraded/pyrogenic compounds (40-74% of preserved organic matter), and class-level composition differed significantly among habitats (PERMANOVA, p < 0.01) while the resistant core was shared. In paired vegetated soils, aliphatic enrichment increased with radiocarbon age, whereas carbohydrates and lignin declined, indicating that the aliphatic-dominated fraction marks diagenetically processed carbon. Statistical modeling identified plant-derived C and SOC stock as the variables most closely associated with molecular diversity in vegetated soils, whereas active mineralogy was the strongest correlate in mudflats. These results indicate that carbon stock, turnover time, and molecular composition represent three independent axes of coastal carbon cycling. Input volume sets the stock, mineral protection and allochthonous contributions set the turnover time, and diagenesis shapes the surviving composition. Unvegetated tidal flats belong in blue carbon inventories, and molecular composition can complement area- and stock-based accounting as an indicator of carbon permanence.