Distribution and controlling factors of iron-bound organic carbon across tidal zones in the Yellow River Estuary

Mineral protection by iron oxides is an important mechanism regulating the long-term stabilization of soil organic carbon (SOC) in coastal wetlands. Reactive iron minerals can stabilize organic matter through adsorption and organo-mineral associations, thereby contributing substantially to ecosystem carbon sequestration. However, coastal wetlands are characterized by strong tidal gradients in salinity, inundation frequency, and redox conditions, which may alter iron mineral transformation and Fe-associated carbon preservation. Despite the importance of these processes for blue carbon stability, the distribution of iron-bound organic carbon (Fe-OC) across tidal zones and its environmental controls remain poorly understood. Here, we investigated the spatial distribution and controlling factors of Fe-OC across natural tidal zones in the Yellow River Delta, China. Soil samples were collected from representative supratidal and intertidal habitats, including the supratidal zone (Stz), high tidal zone I (HtzI), high tidal zone II (HtzII), and middle tidal zone (Mtz), along a natural land-sea gradient. Soil physicochemical properties, iron oxide fractions, and Fe-OC contents were determined to evaluate how tidal-driven environmental variation regulates iron-associated carbon stabilization. The contribution of Fe-OC to SOC (f_Fe-OC) and OC/Fe molar ratios were further quantified to assess the dominant mechanisms underlying iron-organic associations. Fe-OC contents ranged from 0.24-0.54 g/kg and accounted for 6.58%-16.4% of SOC across tidal zones. Both Fe-OC and f_Fe-OC differed significantly among tidal habitats and showed a clear declining trend from supratidal to intertidal zones along the land-sea gradient, indicating that increasing tidal influence weakens Fe-mediated carbon stabilization. The mean OC/Fe molar ratio across all tidal zones was 0.21, suggesting that adsorption dominated the association between iron oxides and organic carbon. Fe-OC was positively correlated with amorphous iron oxides, highlighting the important role of poorly crystalline iron minerals in SOC preservation. In contrast, Fe-OC showed significant negative correlations with soil pH and electrical conductivity, suggesting that saline-alkaline conditions constrain the stabilization of organic carbon by reactive iron minerals. Random forest analysis further identified pH and EC as the dominant predictors of Fe-OC distribution, emphasizing the importance of tidal-induced geochemical conditions in regulating iron-associated carbon preservation. Together, these findings demonstrate that tidal environmental gradients strongly influence Fe-mediated SOC stabilization in coastal wetlands and suggest that increasing salinization may reduce the persistence of mineral-protected blue carbon pools. This study provides new insights into the mechanisms governing SOC stabilization in estuarine wetlands and improves our understanding of blue carbon sequestration under changing coastal environments.