Role of iron oxides in stabilization of soil organic carbon along vegetation distribution gradient in coastal wetland, Southeastern China.

Iron (Fe) oxide-mediated carbon sequestration (i.e., the formation of iron-bound organic carbon, Fe-OC) is a vital mechanism for the stabilization of soil organic carbon (SOC) in coastal wetlands. However, the spatial dynamics and regulatory mechanisms of Fe-OC along sea-to-land vegetation gradient remain incompletely understood. This study systematically investigated the spatial distribution of soil Fe forms, Fe phases, and Fe-OC across a typical coastal sea-to-land vegetation sequence comprising tidal flats (TF), Spartina alterniflora (SA), mixed SA and Suaeda salsa (SA&SS), Suaeda salsa (SS), and Phragmites australis (PA). The results indicated a net accumulation of the total iron pool (including Fe_o, Fe_d, and Fe_p), and altered soil redox conditions along the gradient. The Fe-OC content in the 0-60 cm soil profile ranged from 0.17 to 0.51 g/kg. Specifically, the SA community showed the highest average Fe-OC content and fFeOC, followed by the PA community, which collectively accounted for 20%-30% of the SOC pool. In contrast, the unvegetated TF exhibited the lowest Fe-OC content and the lowest fFeOC (<15%). Vertically, Fe-OC enrichment was most pronounced in the 10-30 cm soil layer. Model analysis further confirmed that mineral-associated organic carbon (MAOC), belowground biomass (BGB), specific surface area (SSA), silt content, Fe_p, Fe_d, and the Fe(III)/Fe(II) ratio were the core positive predictors driving the spatial heterogeneity of Fe-OC. This study highlights that the coupling of vegetation characteristics, nutrient availability, and iron-mineral protection mechanisms regulates the formation of the coastal "rusty sink", thereby providing a crucial scientific basis for enhancing blue carbon stability under climate change.