Soil organic carbon stocks and stabilization mechanisms in tidal marshes along estuarine gradients

• SOC stocks decrease from freshwater to salt marsh. • MAOM, the largest SOC fraction, also decreases from freshwater to salt marsh. • SOC stabilization reflects sedimentation dynamics along the Elbe Estuary. • Lower flooding frequency and salinity favor SOC protection by aggregates. • Higher free POM proportion under reducing soil conditions. Tidal marshes in estuaries store large amounts of soil organic carbon (SOC) and are dominated by the interaction of tidal inundation and salinity with biotic ecosystem components, leading to strong spatio-temporal heterogeneity. Little is known how these estuarine conditions affect SOC stabilization. Our aim was to assess (i) SOC stocks, (ii) SOC stabilization mechanisms (aggregation and mineral-association), and (iii) their environmental drivers along estuarine gradients. We analyzed SOC stocks and SOC density fractions in topsoil (0–10 cm) and subsoil (10–30 cm) of three marsh zones representing three flooding regimes (daily, monthly, yearly) in three marsh types along the salinity gradient (salt, brackish, freshwater) of the Elbe Estuary, Germany. The SOC stocks in 0–30 cm ranged between 9.3–74.6 t ha −1 , and decreased with increasing salinity and flooding. This was linked to decreasing plant biomass and soil fine texture. Organic matter (OM) associated with minerals (C MAOM ) constituted the largest SOC fraction (59 % of SOC), followed by aggregate-occluded OM (C oPOM ) (24 %) and free particulate OM (C fPOM ) (16 %). The C MAOM amount in topsoils decreased with increasing salinity, reflecting decreasing soil fine texture. Amounts of C oPOM were higher in topsoils and high marshes, indicating negative effects of flooding on aggregation. The contribution of C fPOM to total SOC increased with increasing flooding, likely related to its preservation under reducing soil conditions. Our results emphasize that increasing marine influence (rising salinity and flooding frequency) leads to a decrease in SOC content and stabilization. Therefore, sea–level rise has the potential to negatively impact SOC storage in estuarine marshes.