Soil Moisture and Nutrients Regulate Bacterial Communities in Chromite Mining Restoration Areas on the Qinghai–Tibet Plateau

Mining activities on the Qinghai–Tibet Plateau (QTP) have resulted in remarkable damage to the local ecosystem. The ecological restoration of these mining areas is a slow and intricate process, and the re-establishment of soil microorganisms is one of its most important aspects. However, previous studies have seldom investigated the impact of chromite mining on soil organisms, nor the variations in these populations across different ecological restoration durations. In this study, we investigate the impact of soil physicochemical properties on soil microorganisms in chromite mining areas and analyze the variations between recently restored areas (<5 years, NMs) and established restoration areas (>10 years, OMs). The results indicate that (i) chromite mining significantly increases soil bulk density (SBD), pH, electrical capacity (EC), total chromium (TCr), and hexavalent chromium (Cr(VI)), while decreasing soil nutrient levels, including total and available C, N, and P (TC, TN, TP, SOC, AvN, and AvP). These impacts are particularly pronounced in NMs, where the concentrations of TCr and Cr(VI) are 1430.4 ± 421.4 mg kg−1 and 12.62 ± 5.73 mg kg−1, respectively. With increasing ecological restoration duration, TCr and Cr(VI) decreased by 85.4% and 91.4% in OMs (p < 0.05). (ii) Chromite mining decreases the abundance and alpha diversity of soil microbial communities, leading to larger populations of the genera Pseudarthrobacter and Gaiella than in control areas. These effects are also most pronounced in NMs and mitigated in OMs. Soil moisture and nutrients are the primary positive factors driving the soil microbial community assembly. (iii) The decline in soil microbial community abundance and increase in Cr(VI) concentration in former mining areas cause a decrease in the enzymatic activities of amylase, urease, and alkaline phosphatase, resulting in a nutrient-deficient environment, particularly in NMs, and less so in OMs. These findings can provide a reference for the ecological restoration of former chromite mining areas on the QTP.