The preservation of organic carbon by active iron oxides in surface sediments from Guangxi mangroves

Iron oxides play a significant role in the global soil (sediment) organic carbon (OC) storage. Mangrove wetlands, receiving both terrestrial and marine inputs, provide a unique habitat for the preservation of organic carbon by reactive iron oxides (Fe_R). However, the impact of FeR in the surface sediments of mangroves on the preservation process of OC, as well as the selectivity of Fe_R towards different OC components, is still unclear. The surface sediments in the present work was collected in the natural mangrove areas around the estuaries of the Dafeng River and Maowei Sea in Guangxi, which are highly influenced by tides and rivers. The research was focused on the changes in the content and composition of iron-bound organic carbon (Fe-OC) in sediments. The results indicated that the average content of Fe-OC in surface sediments in the estuaries of Dafeng River and Maowei Sea was 0.16% ± 0.07% and 0.17% ± 0.07%, respectively. These values represented 16.2 ± 5.04% and 10.9 ± 5.63% of the total organic carbon (TOC) content in the respective sediments, which were primarily preserved through adsorption. Furthermore, the preservation of OC by Fe_R was influenced by the sediment particle size, the content and form of Fe_R, and the source and composition of TOC. TOC, Fe_R and Fe-OC were mainly distributed in the smaller grain size sediment fractions. The predominant form of iron in surface sediments was Fe³⁺, accounting for 87.42% of the total iron, and was relatively higher in high salinity sediments. Fe_R selectively preserved OC with higher δ¹³C (stable carbon isotope natural abundance) and aromatic OC. Compared with the Maowei Sea, the proportion of protein-like fluorescent components in Fe-OC of surface sediments from the Dafeng River estuary was higher and the proportion of protein-like fluorescent components increased as the molar ratio of Fe-OC∶Fe increased. This study helps to clarify the selective preservation mechanism of OC by Fe_R in mangrove surface sediments in Guangxi, and deepen our understanding of the preservation process of OC in land-sea interface sediments.