The copper spoil heap Knappenberg, Austria, as a model for metal habitats - Vegetation, substrate and contamination

The adaptability, distribution, ecological function and restoration application of biological soil crusts on metal tailings: A critical review

A large amount of metal tailings causes many environmental issues. Thus, the techniques for their ecological restoration have garnered extensive attention. However, they are still in the exploratory stage. Biological soil crusts (BSCs) are a coherent layer comprising photoautotrophic organisms, heterotrophic organisms and soil particles. They are crucial in global terrestrial ecosystems and play an equal importance in metal tailings. We summarized the existing knowledge on BSCs growing on metal tailings. The main photosynthetic organisms (cyanobacteria, eukaryotic algae, lichens, and mosses) of BSCs exhibit a high heavy metal(loid) (HM) tolerance. BSCs also have a strong adaptability to other adverse conditions in tailings, such as poor structure, acidification, and infertility. The literature about tailing BSCs has been rapidly increasing, particularly after 2022. The extensive literature confirms that the BSCs distributed on metal tailings, including all major types of metal tailings in different climatic regisions, are common. BSCs perform various ecological functions in tailings, including HM stress reduction, soil structure improvement, soil nutrient increase, biogeochemical cycle enhancement, and microbial community restoration. They interact and accelerate revegetation of tailings (at least in the temperate zone) and soil formation. Restoring tailings by accelerating/inducing BSC formation (e.g., resource augmentation and inoculation) has also attracted attention and achieved small-scale on-site application. However, some knowledge gaps still exist. The potential areas for further research include the relation between BSCs and HMs, large-scale quantification of tailing BSCs, application of emerging biological techniques, controlled laboratory experiments, and other restoration applications.

Site-specific ecological effect assessment at community level for polymetallic contaminated soil

Current ecological risk assessment (ERA) is based more on book-keeping than on science especially for terrestrial ecosystems due to the lack of relevance to real field. Accordingly, site-specific ecological effect assessment is critical for ERA, especially at high tiers. This study developed procedures to assess ecological effect at community level based on field data. As a case study, we assessed ecological effect of polymetallic contamination in soil in the surrounding of an abandoned mining and smelting site in Hunan, China. Firstly, Zn was identified as the dominant contaminant in soil and slope gradient (SG) and pH as environmental impact factors using distance-based redundancy analysis(db-RDA). Secondly, sensitive endpoints were screened using correlation analysis between Zn and parameters of plant community composition and functional traits. Thirdly, exposure-effect curves between Zn and screened endpoints were developed by taking SG and pH as covariates using Bayesian kernel machine regression analysis (BKMR), based on which half-effect concentrations (EC₅₀s) and 10 %-effect concentrations (EC₁₀s) of soil Zn for each endpoint were calculated. Finally, site-specific hazardous concentrations (HC₅₀s) of Zn were estimated. It was revealed site-specific EC₅₀s and EC₁₀s for soil Zn ranged 80.5-201 mg kg^-1 and 342-893 mgkg^-1, respectively, and HC₅₀s based on EC₁₀s and EC₅₀s ranged 104-110 mg kg^-1 and 595-612 mg kg^-1, respectively, which are more specific and inclusive than those obtained based on crop and vegetable seed germination and seedling growth toxicity experiments.

Structural traits of leaf epidermis correspond to metal tolerance in Rumex acetosella populations growing on metal-contaminated soils

The pseudometallophyte Rumex acetosella L. occupies habitats with normal and high soil concentrations of zinc (Zn), lead (Pb), and copper (Cu). It remains unclear if the plants respond to the toxic metals by altering their morphology and increasing the resilience of their cells. We compared plants growing on soils contaminated with Zn/Pb (populations Terézia, Lintich), or Cu (populations Špania Dolina, Staré Hory), with those from non-contaminated soil (Dúbravka) in Slovakia, and analysed leaf structure, physiology, and metal contents by light and electron microscopy, element localization by energy-dispersive X-ray analysis (EDX) in scanning electron microscope, and by specific fluorescence dyes. In control population, the epidermis of the amphistomatic leaves of R. acetosella contained capitate glandular trichomes, consisting of four head (secretory), two stalk, and two basal cells. The ultrastructure of secretory cells revealed fine wall ingrowths bordered by plasma membrane protruding into the cytoplasm. The metallicolous populations had higher contents of Zn and Cu in the epidermal and glandular cells, and a higher density of both stomata and trichomes. Extensive cell wall labyrinth was present in the trichome secretory cells. Their abnormal number and elevated metal contents might indicate effects of heavy metals, especially of Cu, on mitosis and cell plate formation. Differences in leaf physiology were indicated by significantly higher cytoplasmic tolerance to Zn and Cu in metallicolous populations and by structural properties of glandular heads suggesting secretion of toxic metals. Our findings are suggestive of plant reactions to metal stress, which facilitate the populations to occupy the metal-contaminated sites.

Lamina Cell Shape and Cell Wall Thickness Are Useful Indicators for Metal Tolerance-An Example in Bryophytes

Bryophytes are widely used to monitor air quality. Due to the lack of a cuticle, their cells can be compared to the roots of crop plants. This study aimed to test a hypothetical relation between metal tolerance and cell shape in biomonitoring mosses (Hypnum cupressiforme, Pleurozium schreberi, Pseudoscleropodium purum) and metal sensitive species (Physcomitrium patens, Plagiomnium affine). The tolerance experiments were conducted on leafy gametophytes exposed to solutions of ZnSO4, ZnCl₂, and FeSO₄ in graded concentrations of 1 M to 10⁻⁸ M. Plasmolysis in D-mannitol (0.8 M) was used as a viability measure. The selected species differed significantly in lamina cell shape, cell wall thickness, and metal tolerance. In those tested mosses, the lamina cell shape correlated significantly with the heavy metal tolerance, and we found differences for ZnSO4 and ZnCl₂. Biomonitoring species with long and thin cells proved more tolerant than species with isodiametric cells. For the latter, “death zones” at intermediate metal concentrations were found upon exposure to ZnSO4. Species with a greater tolerance towards FeSO₄ and ZnSO₄ had thicker cell walls than less tolerant species. Hence, cell shape as a protoplast-to-wall ratio, in combination with cell wall thickness, could be a good marker for metal tolerance.

Factors of variation in beech forest understory communities on waste heaps left by historical Zn-Pb ore mining

The species trait-environment relationships in understory vegetation were carried out on metalliferous sites created by historical Zn-Pb mining (S Poland), on which natural revegetation took place. The study sites were 31 small heaps of waste rock covered by an 80-120 year old beech forest. The sites were described in terms of plant coverage, species richness and composition, and the composition of plant traits. Three types of understory communities that were identified were compared for environmental variables and plant parameters. Despite a high concentration of heavy metals in soils, namely Cd, Pb, and Zn, the degree of shading by the tree canopy was the main factor determining variation in understory communities. The understories that developed in low and strong shading were similar to some extent: They had high number of species, and they were characterized by a high proportion of the ancient forest species. The species composition changed substantially with increasing coverage of trees; mixed strategy plants withdrew, while the stress tolerant species became more abundant. The other predictor of community structure was the ratio of Cd to Ca, which was responsible for the decrease of woody plants and the endangered species. This study proved that, in the case of highly complex and productive ecosystems, even with a high degree of contamination with heavy metals, the biotic factors play a primary role.

Waste heaps left by historical Zn-Pb ore mining are hotspots of species diversity of beech forest understory vegetation

Metalliferous mining and smelting industries are associated with very high levels of heavy metal(loid) contamination of the environment. Heavy metals have been proved to significantly influence the species diversity and composition of grassland communities, but little is known on their effects on forest understory vegetation. Therefore, the aim of this study was to investigate the effects of the presence of small heaps of waste rock left by historical Zn-Pb ore mining on understory vegetation. The heaps are scattered over vast areas of beech forests in southern Poland. Three types of study plots were established: (1) on waste heaps themselves, (2) in their vicinity (5-10m from the foot of the heaps, with no waste rock but potentially influenced by the heaps through drainage water), and (3) at least 100m from the foot of the heaps (pseudo-control). In all plots vegetation parameters, i.e., plant species number, cover and community composition, life forms and strategies, as well as basic soil properties were assessed. Although the heaps contained high concentrations of metals, namely Cd, Pb and Zn, they were characterised by higher cover and diversity of understory vegetation, including ancient forest and endangered species, in comparison to their surroundings. They were also characterised by the distinct species composition of their plant communities. This might have resulted from the beneficial influence of high pH and Ca content originating from waste rock composed of dolomite and calcite, as well as from increased habitat heterogeneity, e.g. soil skeleton and steeper slopes. Another important factor influencing the richness and composition of understory was tree cover, which relates to the light transmissibility of the canopy. Our study proved that the disturbance brought about by the former mining and processing of metal ores led to the formation of species-rich understory with high frequency and cover of naturally-valuable species.