Changes in soil properties in the soil profile after mining and reclamation in an opencast coal mine on the Loess Plateau, China

Surface mining involves drastic disturbances to regional ecosystem and soil properties. Recovery of soil physicochemical characteristics is essential for successful restoration of the landscape and the soil itself after mining. To identify the changes in the soil profile after mining and reclamation, we studied the soils of the largest opencast coal mine in China, Pingshuo, located in the Loess Plateau. Soil samples representing three different land use types and six different reclamation times were collected in the soil profiles (0-100 cm) in 2012 and were analyzed for field capacity, bulk density, pH, soil organic matter (SOM), N, P, K, available P, and available K.Results showed that most reclaimed mine soil properties (including bulk density, pH, P, K, available P and available K) increased in comparison with those of the natural soils, whereas SOM and N decreased after mining and reclamation, especially in the topsoil (0-40 cm). Trend lines of P and available K in the reclaimed mine soils and natural soils were similar tested by Anova. Trend lines of P in the natural soils, and pH, P and K in the reclaimed mine soils were relatively stable and showed no difference in the soil profile (P < 0.05). Soil properties and their trend lines in forest land were more similar to natural soils than were cultivated land and grass land in deeper (60-100 cm) soil layers. Reclamation time (time that has passed after reclamation was finished) was positively associated with SOM, N, P, and available P in cultivated land; pH decreased with the increase of reclamation time in forest land but N showed the opposite trend; pH and P were positively correlated with reclamation time but K and available K showed negative relationships with reclamation time in grassland. Bulk density vs. field capacity and available K vs. pH showed significant correlations in the soil profile (0-100 cm) (P < 0.05). Significant correlations were also observed for available K vs. bulk density in the topsoil layer (0-30 cm) and bottom layer (below 60 cm) (P < 0.05). P and pH showed positively significant correlation in the subsoil layer (30-60 cm) and bottom layer (below 60 cm) (P < 0.05). Most of heavy metals (As, Cd, Cr, Cu, Pb and Hg) of cultivated land exceeded their corresponding background values in Shanxi province, where our sites were located. However, compared with Chinese soil guidelines (Grade II values) (GB15618-1995), heavy metals were all below the threshold values.