Temporal dynamics of minesoil restoration in Southern Brazil: A two-decade monitoring of soil physical attributes and organic carbon content

Brazil covers a huge territory (852 Mha) with biomes distributed from North to South of the country. In Southern Brazil, about 24% of the area covered by the Pampa Biome was lost between 1985 and 2022, mainly due to conversion to agriculture, but also due to coal mining. This region holds the largest coal mine in Latin America, namely Candiota Mine, which represents 38 % of the Brazilian coal reserves. Minesoils typically exhibit severe compaction and poor structure, jeopardizing its reclamation and reintegration to the biome. This study aimed to assess the temporal changes in physical attributes and total organic carbon (TOC) content of a minesoil as affected by perennial grasses in a long-term field randomized complete block design experiment located in the Candiota Mine region. Based on prior research indicating superior performance of Urochloa brizantha for amelioration of this minesoil, we hypothesized a continued outstanding performance of this grass over the others to enhance the structure and increase the TOC content of the minesoil. Experimental treatments (with four replicates) consisted of perennial grasses used for minesoil revegetation: Hemarthria altissima, Paspalum notatum, and Urochloa brizantha. Minesoil bulk density (Bd), macroporosity (Ma), total porosity (Tp), percentage of macroaggregates and microaggregates, and TOC content were evaluated in the different treatments at 0.00−0.10 and 0.10−0.20 m layers at four restoration time points (0.5, 8.6, 14.6 and 20 years). The elevated percentage of macroaggregates and Bd close to or > 1.50 Mg m⁻³ observed for the 0.00−0.10 and 0.10−0.20 m layers up to 8.6 years reflect the persistent compaction caused by the traffic of heavy machinery on the minesoil during landscape restoration. Minesoil compaction was remarkably attenuated after 14.6 years of restoration, likely due to root-induced disruption of aggregates formed by compression followed by soil re-aggregation. This was consistent with a reduction of Bd and an increase of Ma and TOC of the minesoil at this same restoration age. Urochloa brizantha was the pioneer grass alleviating minesoil compaction but this outstanding performance leveled off after 14.6 years compared to the other grasses, contracting our hypothesis. This was confirmed by the ordination of Urochloa brizantha close mainly to Hemarthria altissima in a principal component analysis biplot of the data set, indicating similar interrelations of the examined attributes after 20 years of restoration, regardless of the grass species.