Soil physical recovery drives organic carbon accumulation during long-term grass revegetation in coal mine soils

Vegetative restoration with perennial grasses is applied for the rehabilitation of coal mining areas in southern Brazil; however, soil compaction may limit organic carbon accumulation. This study evaluated the influence of soil physical quality on total organic carbon (TOC) in a coal mine soil revegetated with perennial grasses over more than a decade of recovery. Soil samples were collected to determine bulk density (Bd), macroporosity (Ma), total porosity (Tp), and TOC using a factorial experimental design including revegetation time (2.0, 4.8, 10.6, and 16.2 years) and grass species (Urochloa brizantha, Panicum maximum, and Urochloa humidicola). Restoration time exerted a strong influence on soil physical properties and TOC, regardless of grass species. In the 0.00-0.10 m layer, bulk density decreased from 1.41 Mg m⁻³ at 2.0 years to 1.21 Mg m⁻³ at 10.6 years, while macroporosity increased from 0.09 to 0.20 m³ m⁻³ and total porosity from 0.47 to 0.55 m³ m⁻³. Similar but less pronounced trends were observed in the 0.10-0.20 m layer. Total organic carbon increased with restoration time, reaching maximum values at 10.6 years (35.30 g kg⁻¹ in the 0.00-0.10 m layer), followed by a decline at 16.2 years (23.81 g kg⁻¹). Grass species exhibited similar responses for soil physical attributes and TOC, indicating that restoration time was more influential than species selection. Overall, results demonstrate that improvements in soil physical quality are a key driver of organic carbon accumulation in revegetated coal mine soils.