Soil organic matter dynamics across aggregates and granulometric fractions in coal-mined soils under grass revegetation

Open-pit coal mining leads to vegetation removal and severe soil disturbance, directly promoting losses of soil organic carbon. Although revegetation is widely applied to restore mined areas, important knowledge gaps remain regarding the mechanisms of carbon stabilization, which involve root system expansion and the slow recovery of soil biological processes, even after vegetation is established. This study aimed to evaluate the distribution of soil organic matter fractions across different aggregate size classes after 20 years of revegetation with perennial grasses in a coal-mining area in southern Brazil. The proportions of macro, meso, and microaggregates were assessed in the 0.00–0.10 and 0.10–0.20 m soil layers of a minesoil revegetated with Hemarthria altissima, Cynodon dactylon cv. Tifton, and Urochloa brizantha. Within each aggregate class, soil organic matter was fractionated into particulate and mineral-associated organic matter. Data were analyzed using analysis of variance (ANOVA) followed by Tukey’s test (p < 0.05). After 20 years, the 0.00–0.10 m layer showed a predominance of macroaggregates (mean of 47.6%) under all grass species, with no significant differences among them. In the 0.10–0.20 m layer, Urochloa brizantha exhibited a higher proportion of macroaggregates (42.6%). No significant differences were observed in particulate organic matter and mineral-associated organic matter contents among aggregate classes or grass species. However, mineral-associated organic matter was the dominant fraction across all aggregate classes, indicating its key role in carbon stabilization in revegetated minesoils. These results suggest that severely degraded minesoils, once restored, may offer relevant potential for long-term soil carbon storage.