Properties of iron-bound organic carbon and its implications for the conservation of coastal wetland vegetation

Reactive iron oxides, as an efficient “rust sink” for organic carbon, play a pivotal role in the long-term preservation of organic carbon within global soils. Although coastal wetlands are crucial carbon sinks on Earth, the composition of reactive iron-bound organic carbon (Fe_R-OC) remain unclear. In this study, we applied a modified citrate-bicarbonate-dithionite (CBD) extraction method coupled with advanced analytical techniques including optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and stable isotope mass spectrometry to investigate Fe_R-OC in the Yellow River coastal wetland in China. Our findings reveal that the Fe_R-OC:Fe_R ratios are relatively low (0.1–0.8), suggesting that adsorption is the primary mechanism controlling Fe_R-OC formation in the Yellow River coastal wetland. Correlation analysis between Fe_R content and fluorescence components indicates that iron oxides preferentially adsorb biologically recalcitrant humic-like components, while exhibiting limited affinity for protein-like. Meanwhile, we identified 1440 dissolved organic matter (DOM) molecules adsorbed by iron oxides, predominantly by oxygen-rich and highly unsaturated molecules. Furthermore, the Fe_R-OC content in vegetated areas is an order of magnitude higher than bare flat, indicating that the restoration of vegetation is effective strategy for enhancing carbon sequestration in coastal wetlands. This study bridges laboratory simulations with natural samples, establishing a novel protocol enables more precise understanding of Fe-C coupling in real environments.