关键词: 矿山废弃地, 先锋植物, 根际土壤, 理化性质, 酶活性, 重金属积累

Abstract:

[Objective] Open-pit mining dramatically alters the original topography and soil structure, causing severe vegetation damage, soil erosion, and land subsidence. Meanwhile, waste ore and tailings generated during mining often contain heavy metals that accumulate in soils through dissolution, precipitation, and aggregation, thereby causing serious environmental pollution. Therefore, the ecological restoration of mine sites is urgently needed to support sustainable development in China. Vegetation restoration, which harnesses natural plant succession to rebuild degraded mine ecosystems, is considered one of the most effective strategies. Although the high heavy-metal content, poor soil structure, and low fertility of abandoned mining soils restrict plant growth, some pioneer plants can adapt to these harsh conditions. Therefore, selecting pioneer plants with high heavy-metal tolerance and hyperaccumulation ability is essential for mine-site restoration. In this study, we selected a naturally restored mine site in the Shaanxi section of the Qinling Mountains and screened 17 widely distributed pioneer plant species to evaluate their potential for ecological restoration. [Methods] A naturally restored mine site in the Shaanxi section of the Qinling Mountains was selected as the study area. Through field surveys, 17 widely distributed pioneer plant species and their corresponding rhizosphere soil samples were collected. Soil physicochemical properties, including pH, soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), and available phosphorus (AP), were analyzed. The activities of soil enzymes, including urease, sucrase, and alkaline phosphatase, were measured. The concentrations of six heavy metals—Pb, Zn, Mo, Cr, Ni, and As—in soils and plant tissues were determined. The bioaccumulation and translocation characteristics of heavy metals in plants were evaluated using bioaccumulation factors and translocation factors. In addition, random forest models and Mantel tests were jointly used to identify the key environmental factors influencing heavy-metal bioavailability in the rhizosphere soils. [Results] The pH values of the rhizosphere soils of all pioneer plants ranged from 7.00 to 8.52, indicating weakly alkaline conditions, and were generally lower than that of bare soil. Rhizosphere nutrient contents also differed significantly among plant species. The rhizosphere soil of P. scabiosifolia had the highest SOC content (19.25 g·kg⁻¹). The highest AN contents were observed in the rhizosphere soils of P. chinensis (10.64 mg·kg⁻¹) and C. vlassovianum (9.50 mg·kg⁻¹). The rhizosphere soil of V. prionantha also had relatively high TN (6.11 g·kg⁻¹) and AN (6.32 mg·kg⁻¹) contents. A. argyi had the highest rhizosphere AP content (17.44 mg·kg⁻¹). The activities of urease, sucrase, and alkaline phosphatase in the rhizosphere soils of all pioneer plants were significantly higher than those in bare soil. The urease activities were highest in the rhizosphere soils of A. vitifolia (1.76 mg·g⁻¹·h⁻¹), V. prionantha (1.47 mg·g⁻¹·h⁻¹), and C. amabile (1.43 mg·g⁻¹·h⁻¹), exceeding the activity in bare soil (0.07 mg·g⁻¹·h⁻¹) by more than 20-fold. Sucrase activities in the rhizosphere soils of C. lavandulifolium (4.73 mg·g⁻¹·h⁻¹), P. scabiosifolia (3.83 mg·g⁻¹·h⁻¹), and V. prionantha (3.76 mg·g⁻¹·h⁻¹) were approximately 20-25 times higher than that in bare soil (0.19 mg·g⁻¹·h⁻¹). Alkaline phosphatase activities in the rhizosphere soils of C. divaricatum (0.04 mg·g⁻¹·h⁻¹), P. fragarioides (0.039 mg·g⁻¹·h⁻¹), and V. prionantha (0.036 mg·g⁻¹·h⁻¹) were approximately 4.5-5.0 times higher than that in bare soil (0.008 mg·g⁻¹·h⁻¹). The concentrations of Pb, Zn, Mo, Cr, Ni, and As in the mine soils exceeded the corresponding soil background values for Shaanxi Province, with Pb, Zn, and Mo contamination being particularly severe. Pioneer plant species differed significantly in their capacities to bioaccumulate and translocate soil heavy metals. The concentrations of Pb and Zn in M. sinensis were significantly higher than those in the other plants. The Zn concentration in its belowground tissues (360.34 mg·kg⁻¹) was 2.4 times that in its aboveground tissues (151.25 mg·kg⁻¹), whereas the Pb concentration in its belowground tissues (819.28 mg·kg⁻¹) was 1.9 times that in its aboveground tissues (425.30 mg·kg⁻¹). Overall, all plant species showed strong capacities to accumulate Ni and Cr in their aboveground tissues. In particular, the aboveground bioaccumulation coefficients (f_s) of P. viristriatus, A. argyi, and V. prionantha all exceeded 0.4 for both Ni and Cr. All plants exhibited weak aboveground accumulation of Mo, As, and Pb, and the f_s values for As were below 0.1 in all species. The belowground tissues of T. aquilegiifolium var. Sibiricum generally contained higher concentrations of all six heavy metals than those of the other plants. The translocation factors (t) of A. vitifolia for Zn, Mo, and Pb were 3.81, 1.67, and 1.73, respectively, and were significantly higher than those of the other plants. The t of V. prionantha for Zn, Mo, and Pb were also relatively high, at 1.27, 1.33, and 1.35, respectively. Random forest analysis of the effects of rhizosphere soil physicochemical properties on the bioavailability of Pb, Zn, Mo, Cr, Ni, and As showed that pH was the dominant factor affecting the bioavailability of Pb and Mo, whereas TN significantly affected the bioavailability of Pb, Zn, Mo, Ni, and As. Mantel tests showed that soil TOC was significantly and positively correlated with TN (p ≤ 0.001) and sucrase activity (p ≤ 0.05). Urease activity was positively correlated with TN (p ≤ 0.05), whereas alkaline phosphatase activity showed a nonsignificant negative correlation with AP (p > 0.05). Cluster analysis showed that C. amabile, T. aquilegiifolium var. Sibiricum, C. lavandulifolium, A. vitifolia, A. argyi, V. prionantha, and P. scabiosifolia performed well in improving soil physicochemical properties and enzyme activities, with P. scabiosifolia showing particularly high rhizosphere nutrient levels. These species showed strong overall capacities for heavy-metal accumulation. In particular, V. prionantha exhibited high heavy-metal translocation and aboveground bioaccumulation capacities, indicating that it is suitable for the phytoextraction of heavy metals. [Conclusion] Considering rhizosphere soil physicochemical properties and enzyme activities, together with plant heavy-metal bioaccumulation and translocation capacities, V. prionantha and P. scabiosifolia showed clear advantages in simultaneously improving mine-soil fertility and accumulating heavy metals. They are therefore recommended as priority species for ecological restoration of mine sites in the study area.

Key words: abandoned mine land, pioneer plant, rhizosphere soil, physicochemical properties, enzyme activity, heavy metal bioaccumulation