Phytostabilization potential and microbial response to the reclamation of native Cynodon dactylon in spoil heaps from a multiple-metal mining site in Southwest China
Description
Phytocapping offers a sustainable approach for managing exposed tailings by mitigating pollutant spread and enhancing phytoremediation. This study investigates the potential of Bermudagrass (Cynodon dactylon) as a pioneering plant for rehabilitating tailings from an open-pit lead-zinc mine in Southwest China. Our findings demonstrate that Bermudagrass significantly improved soil quality and multifunctionality compared to adjacent bare tailings. Soil improvements included increases in organic matter (107%), total and available nitrogen (50% and 110%, respectively), available phosphorus (170%), and soil enzyme activities, including β-glucosidase (170%), sucrase (1729%), alkaline phosphatase (3722%), and acid phosphatase (168%). The reclamation process also promoted microbial community succession, altering community composition, improving microbial diversity, and enhancing bacterial biomass from (0.89 ± 0.54) × 10¹⁵ to (9.06 ± 3.25) × 10¹⁵ copies / g in rhizosphere soils. Greenhouse experiments further confirmed Bermudagrass’s resilience to cadmium (Cd), with both mining and non-mining ecotypes thriving in tailing soils and Cd2+ hydroponic solutions (up to 44.5 μM) without evident phytotoxicity. Bermudagrass roots exhibited exceptional Cd accumulation (bioconcentration factor: 181–1006) while minimizing Cd translocation to shoots (translocation factor: < 0.13). Inoculation with Funneliformis mosseae, a restored root-mutually symbiotic fungus, further mitigated Cd-induced phytotoxicity and enhanced plant growth. These findings highlight Bermudagrass as a promising pioneer species for phytostabilization in severely contaminated mining environments, with its rhizosphere microbiome playing a critical role in facilitating ecosystem restoration. Sustainable plant establishment in mine waste rock requires concurrent development of belowground fertility and healthy rhizospheric soil. Ultimately, successful revegetation depends on integrated above and belowground development to achieve long-term ecological restoration.
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Figure 1/Figure S2: Plant samples and soil samples were digested using a mixture of HNO₃-HClO₄ (4:1, v/v) for plant tissues, and HNO₃-HClO₄-HF (5:1:2, v/v) for soil samples. The total metal concentrations were then measured using a flame atomic absorption spectrometer (Hitachi, FAAS ZA-3000, Japan. The bioavailability of heavy metals in the soils was evaluated through extraction using a diethylenetriaminepentaacetic acid (DTPA) extraction solution. BCF and TF were calculated as: BCF = Croot / Csoil and TF = Cshoot / Croot (Croot, Cshoot, Csoil: total heavy metal concentrations in roots, shoots, and soil). Figure 2: Soil physico-chemical properties and enzyme activity data were standardized using Z-scores. The average Z-score was then calculated to determine the soil multifunctionality index. Figure 3: Alpha diversity metrics were c calculated using Mothur (v1.30.2). The 16S and 18S gene copy numbers of Rs and Bs were quantified using absolute quantitative real-time PCR. Pearson correlation analysis was used to assess the relationship between soil multifunctionality and the alpha diversity index. Figure 4: The relative abundances of dominant species in each sample group were analyzed at the genus level based on bacterial 16S and fungal 18S rRNA genes. Analysis was performed on the Major Cloud platform using default parameters. Species exhibiting significant differences among groups were identified using the Wilcoxon rank sum test. Figure 5: PCoA based on Bray-Curtis dissimilarity (Vegan v2.5-3) was used to evaluate microbial community similarity. The UPGMA algorithm was applied to constructe a dendrogram for hierarchical clustering. The correlation between soil multifunctionality and the β-diversity index was assessed using Pearson correlation analysis. Figure 6: db-RDA (Vegan) was used to assess the influence of microbial communities on soil enzyme activity. Relationships between dominant microbial taxa and key soil properties were examined using MaAslin2. Figure 7: Detection methods for C. dactylon biomass, heavy metal and DTPA-Cd concentrations, as well as BCF and TF, are referenced in Figure 1 and Figure S2. Figure S4: Fresh root subsamples were treated with 5 mL of 10% KOH (w/v) at 90 °C for cleaning, followed by staining with 0.5% acid fuchsin. The colonization status of AMF and DSE was evaluated using the magnified intersection method, with over 300 intersections. Table S1: Detection methods for soil total heavy metal concentrations are referenced in Figure 1 and Figure S2. Soil pH was measured using potentiometric method, following mixing soil with deionized water at 1:25 (w/w) ratio.