Coastal SOM degradation regulates the transformation of iron in mangrove sediments and its environmental significance

Iron (Fe), a key micronutrient, plays a vital role in governing the health and services of mangrove ecosystems. However, the migration and transformation of Fe induced by sedimentary organic matter (SOM) in mangrove wetlands remains unclear. Here, we investigated the Fe biogeochemical processes and effects of SOM components on Fe behaviors in mangrove surface sediments from Dongzhai Harbor, Hainan Island, using sequential extractions of Fe species and spectroscopic techniques. The results showed that most of the Fe species were amorphous oxides (Fe_OX1), crystalline (Fe_OX2), and ferrosilicate (Fe_PRS) in mangrove sediments. The EEM-PARAFAC model successfully identified four fluorescent components: three humic-like components (C1, C2, and C3) and one protein-like component (C4). Terrestrial humic substances have the potential to immobilize Fe_OX1, Fe_OX2, magnetite, and Fe_PRS through adsorption, complexation, and co-precipitation. Microbial degraded humic-like materials may facilitate Fe transformation in the presence of microorganisms while concurrently engaging in the cycling of nitrogen and phosphorus. The processes potentially lead to increased nutrient levels in the overlying water and eutrophication. These findings offer insights into the Fe-SOM couple biogeochemical cycling processes in mangrove sediments, further providing valuable guidance for the ecological conservation of mangrove wetlands.