Mineralogical and organomineral controls on soil organic carbon stabilization across humid tropical biomes impacted by gold mining

Illegal gold mining causes severe soil degradation through vegetation removal, profile disturbance, and accelerated oxidation of soil organic matter, resulting in substantial carbon losses and weakened organomineral stabilization. These effects differ among biomes because mineralogical and biogeochemical processes control carbon retention and persistence in the clay fraction. In this study, we carried out batch sorption experiments with dissolved organic matter (DOM) extracted from Brachiaria brizantha to evaluate how soil mineralogical constitution regulates the retention, spatial distribution, and chemical composition of SOM impacted by mining in the Amazon, Cerrado, and Atlantic Forest Brazilian biomes. ATR-FTIR analysis indicated that DOM consisted of a mixture of labile and functionalized compounds, dominated by polysaccharides, aromatic structures, and carboxylic groups. Sorption experiments revealed biome-specific contrasts, with Amazonian soils exhibiting the highest carbon retention, followed by Atlantic Forest and Cerrado soils. STXM-NEXAFS spectromicroscopy demonstrated that clay-associated carbon in Amazonian soils was enriched in O-alkyl and carboxylic groups, indicating oxidized and stabilized organic matter. In the Atlantic Forest soil, functionalized carbon predominated, reflecting intense biogeochemical cycling and strong mineral interactions. Conversely, Cerrado soils exhibited a higher contribution of carbonates and less functionalized carbon, suggesting lower organomineral stabilization efficiency. Nano-XRF mapping showed spatial co-localization of sulfur and iron in Amazonian and Atlantic Forest soils, while nano-XRD confirmed goethite and hematite as key mineral phases involved in carbon protection. Overall, the results indicate that recarbonization strategies for mined tropical soils should be adapted to local biome and mineralogical conditions, prioritizing iron oxyhydroxide-rich systems to enhance carbon sequestration and persistence.