Reclamation promotes the succession of the soil and vegetation in opencast coal mine: A case study from Robinia pseudoacacia reclaimed forests, Pingshuo mine, China

Robinia pseudoacacia is the dominant tree species in opencast coal mine reclamation in the Loess Plateau, China. Nevertheless, the long-term changes of reclaimed mine soils (RMSs) and vegetation in the reclaimed sites under R. pseudoacacia monoculture forest have not been sufficiently elucidated. In this study, the RMSs physicochemical properties were measured and vegetation (including tree, herb and reserved litter) indexes were surveyed in R. pseudoacacia monoculture forests at 7 reclamation ages (8-, 10-, 13-, 15-, 18-, 23- and 30-year) in the Pingshuo opencast coal mine, China. Adjacent, unreclaimed plot (hereafter called 0-year reclamation plot) was selected as the base line to detect the dynamics of RMSs properties and undisturbed site was chosen to compare with the reclaimed ones. The results showed that, pH (varied from 8.72 to 8.07) and soil bulk density (BD) (ranged from 1.62 to 1.36 Mg m−3) decreased along reclamation chronosequence. In contrast, soil organic matter (SOM, differed from 3.97 to 35.17 g kg−1), total nitrogen (TN, differed from 0.23 to 1.19 g kg−1) and available potassium (AK, ranged from 62.87 to 233 mg kg−1) were found to increase sharply (P < 0.05) in the initial phase of reclamation and tended to be stagnant in the subsequent stage. Moreover, available nitrogen (AN, varied from 7.07 mg kg−1 to 82.20 mg kg−1) was observed to follow a concave curve along reclamation chonosequence. Total phosphorus (TP, differed from 0.48 g kg−1 to 0.60 g kg−1), available phosphorus (AP, varied from 4.33 mg kg−1 to 8.3 mg kg−1) and total potassium (TK, varied from 14.23 g kg−1 to 19.37 g kg−1), however, fluctuated at steady conditions after reclamation and remained the undisturbed levels throughout the reclamation stages. The duration required to attain the nutrient level in undisturbed soils was speculated 10 years of reclamation in the Pingshuo mine. Tree indexes (i.e., diameter at breast height, height, canopy area, biomass and biomass density) increased with the duration of reclamation. Herb indexes (including height, coverage percentage and biomass density), however, followed inversed U functions. Conversely, reserved litter biomass decreased progressively over time. Vegetation indexes were found significantly correlated with SOM, TN, AN and AK (P < 0.05). Overall, R. pseudoacacia monoculture forests markedly ameliorated both soil and vegetation succession in the Pinshuo dumps.