Preservation of soil organic carbon in coastal wetlands promoted by glomalin–iron–organic carbon ternary system

Abstract Iron‐bound soil organic carbon (Fe‐bound SOC) and microbial residues are vital for SOC preservation, a process crucial for drawdown of anthropogenic carbon. However, the relative importance of Fe‐bound SOC in coastal wetlands, and its interaction with microbial residues remain poorly understood. Here, we used glomalin as a proxy for microbial residues, with the goal of determining microbial residues binding to Fe and Fe‐bound SOC across a 1000 km coastline of China including seven typical mangrove wetlands. As hypothesized, it was found that the glomalin fraction was strongly associated with and likely bound to Fe‐bound SOC complexes, which accounted for 21.4% ± 1.6% of mangrove SOC. Mean level of Fe‐bound glomalin was 1.2 ± 0.1 mg cm −3 , accounting for 40.6% ± 1.5% of total glomalin and 35.4% ± 3.2% of Fe‐bound SOC in the top 10 cm mangrove soil layer at regional scale. Due to the growing realization that Fe may be a crucial factor of glomalin cycling, we performed multiple chemical characterization techniques to identify the association between Fe and glomalin. Our results suggested that association to Fe was a main stabilization mechanism of glomalin in soils, and glomalin served as organic ligands in the formation of Fe‐bound SOC. We also found that the bulk glomalin extracts were a glomalin–Fe–OC ternary complex, which constituted an important proportion of mangrove SOC. These findings highlight the important effect of glomalin bound to Fe as a major mechanism of SOC persistence in coastal wetlands.