Spatial distribution and environmental drivers of iron-bound organic carbon in coastal wetlands across climatic gradients

Abstract

The biogeochemical coupling of iron (Fe) and organic carbon (OC) in coastal wetlands is a crucial yet underexplored process for blue carbon stabilization, with iron-bound organic carbon (Fe-OC) serving as an efficient preservation mechanism due to its resistance to degradation. Despite its significance, large-scale studies investigating the latitudinal distribution of Fe-OC and its environmental drivers across varying climatic regimes remain limited. To address this gap, we integrated field surveys, isotopic analyses, and multivariate modeling across Spartina alterniflora-dominated wetlands spanning 14° latitude in eastern China to investigate Fe-OC variability and primary drivers. The results showed that Fe-OC concentrations ranged from 0.17 to 2.13 mg g^–1, showing a significant decline in concentration as latitude increases. Fe-OC concentrations were highest in the surface layer (10–20 cm) and decreased with depth, indicating a vertical stratification pattern. Fe-OC contributed 8.74 % to 41.67 % of soil organic carbon (SOC), with a molar OC/Fe ratio of 1.14 ± 0.69, indicating adsorption as the primary binding mechanism between reactive iron oxides and organic matter. δ¹³C isotopic analyses revealed that Fe-OC was enriched in ¹³C compared to SOC, indicating a higher proportion of marine-derived organic carbon inputs. Multivariate analyses identified key climatic factors, such as temperature and precipitation, along with soil properties (pH, clay content, and concentrations of calcium and aluminum ions) as the primary factors controlling Fe-OC storage. By clarifying how Fe-OC deposition and distribution shift with climate and revealing the iron-mediated processes that stabilize blue carbon, this study provides a data-driven basis for monitoring and targeted management of coastal-wetland blue carbon under future climate scenarios.