Mineral amendments drive Compost Humification and Mineral-Associated Organic Carbon Formation in Acidic Soil

The need to valorize organic wastes and strengthen soil carbon sinks has highlighted mineral-organic matter (OM) interactions as key regulators of compost humification and soil carbon transformation. This study evaluated the effect of five minerals (shale, calcite, kaolinite, sepiolite, and hematite) on OM transformation during vegetable-residue composting and redistribute soil organic carbon (SOC) fractions after soil application. Composting and acidic soil incubation experiments were used to characterize compost humification and organo-mineral complexes (OMCs) formation, and to assess soil properties, enzyme activities, and SOC fractions after compost application. Results showed that mineral amendments accelerated compost humification. Hematite produced the strongest stimulation of humic acid (HA), increasing HA by 1.86-fold. Adsorption experiments showed rapid sorption of dissolved organic matter by minerals, which altered the distribution of mineral-associated organic fractions. The structurally embedded fraction increased by 1.19-1.64-fold, and hematite produced the largest increase in the chemically occluded fraction (1.43-fold). Hematite enhanced carbon stability by preferentially enriching HA within OMCs. Application of mineral-amended composts markedly reshaped SOC partitioning and increased the carbon pool management index by 2.67-3.11-fold. Hematite-amended compost further shifted SOC toward more protected fractions, with mineral-associated organic carbon and non-active organic carbon increasing by 2.51- and 2.78-fold, respectively. These findings show that mineral identity regulates compost maturation and SOC fraction distribution, providing a basis for designing mineral-amended composts for carbon management in acidic soils.