Iron-Doped Biochar Boosting Salt Marsh Blue Carbon via Regulation of Microbial Metabolism and Intensified Mineral-Associated Organic Carbon

Salt marsh ecosystems, with substantial carbon sequestration capacity and ecological value, are experiencing a persistent decline globally. Biochar, especially that derived from Spartina alterniflora, is a promising material for restoration, but conventional biochar has shown limited effectiveness. This study introduces iron-doped biochar (FSBC) as an innovative material that boosts salt marsh blue carbon via regulation of microbial metabolism and intensified mineral-associated organic carbon (MAOC) formation. In experimental cultivation of Sesuvium portulacastrum, the addition of FSBC increased biomass density by 332.4%, carbon stock by 14.0%, and MAOC by 38.3% compared to biochar without iron amendment. These results suggest that FSBC can significantly improve both plant growth and carbon storage in salt marshes. Through microbiome and metabolome analyses, we elucidated the regulatory mechanisms of Fe in plants and its protective role in soil carbon components. FSBC affected bacteria associated with Fe like Pseudomonadales and Croceicoccus, thereby influencing metabolic pathways related to energy transporters and nutrient uptake in plants, including ABC transporters and phenylalanine metabolism. These changes in metabolic pathways promote the accumulation of soil organic acids, enhancing plant iron nutrition. Additionally, FSBC regulates soil Fe components and key microbial communities, promoting Fe-carbon complexation and reducing carbon-related extracellular enzyme activity, thereby enhancing stable carbon accumulation. In conclusion, regulating microbial metabolism and intensifying mineral-associated organic carbon by addition of iron-Doped Biochar significantly enhance blue carbon sequestration in salt marshes. This provides a promising approach for salt marsh restoration.