Effects of afforestation on Technosol properties in reclaimed hard coal deep mining spoil heaps

Mining for fossil fuels and minerals generates spoil heaps and open pits, which have significant environmental impacts in addition to their economic contributions. Afforestation of these disturbed areas can improve soil properties, thereby increasing the functionality and resilience of terrestrial ecosystems. However, the extent of changes in soil properties following afforestation varies depending on the methods used for tree introduction. There is a need for knowledge on the effects of afforestation on soil properties, especially in post-mining Techonosols. Therefore, the objective of this research is to evaluate the effects of three afforestation methods, succession on barren spoil top (SBT), succession on reclaimed topsoil (STS), and plantation on reclaimed topsoil (PTS), on soil properties in a coal post-mining site. Soil samples were collected from 30 randomly established plots (10 × 10 m) for physical and chemical analyses, focusing on the upper layer (0-10 cm depth). The collected samples were analyzed for soil texture, bulk density (BD), porosity, air capacity, capillary water capacity (CWC), moisture content (MC), exchangeable base cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), total organic carbon (SOCt), SOC fractions, total nitrogen (Nt), and total sulfur (St). The results showed that PTS had significantly higher CWC, Nt, Ca²⁺, K⁺, occluded light fraction of carbon (ColF), and mineral-associated carbon fraction (CMAF) compared to SBT. These improvements highlight the effectiveness of active reclamation in enhancing soil structure stability, nutrient retention, and long-term carbon stabilization, critical elements for post-mining ecosystem restoration. In contrast, SBT had greater porosity, Mg²⁺, and free light fraction of carbon (CflF) than STS. In addition, SBT had greater St compared to STS and PTS. This indicates that both natural succession and active restoration contribute to soil change through different mechanisms. Therefore, the choice between afforestation strategies should depend on factors such as restoration objectives, topsoil availability, and resource constraints, as active restoration is labor-intensive and costly.