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Phytomanagement is considered a suitable option in line with nature-based solutions to reduce environmental risks associated to metal(loid) mine tailings. We aimed at assessing the effectiveness of biochar from pruning trees combined with compost from urban solid refuse (USR) to ameliorate the conditions of barren acidic (pH ~5.5) metal(loid) mine tailing soils (total concentrations in mg kg -1 : As ~220, Cd ~40, Mn ~1800, Pb ~5300 and Zn ~8600) from Mediterranean semiarid areas and promote spontaneous plant colonization. Two months after amendment addition were enough to observe improvements in chemical and physico-chemical tailing soil properties (reduced acidity, salinity and water-soluble metals and increased organic carbon and nutrients content), which resulted in lowered ecotoxicity for the soil invertebrate Enchytraeus crypticus. Recalcitrant organic carbon provided by biochar remained in soil whereas labile organic compounds provided by USR were consumed over time. These improvements were consistent for at least one year and led to lower bulk density, higher water retention capacity and higher scores for microbial/functional-related parameters in the amended tailing soil. Spontaneous growth of native vegetation was favored with amendment addition, but adult plants of remarkable size were only found after three years. This highlights the existence of a time-lag between the positive effects of the amendment on tailing soil properties being observed and these improvements being translated into effective spontaneous plant colonization., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Mining activities on metal sulphide ores are considered one of the most environmentally damaging anthropogenic activities worldwide, mainly due to the production of metal(loid)- enriched tailings. The objective of the work was to elucidate which factors, including edaphic, vegetation and microbiological aspects, determine soil functionality in the phytomagement of mine tailings piles in semiarid areas. For this purpose, a field study was conducted using a transect experimental design in a former mining area located in southern Spain. Soil characteristics, including edaphic parameters, bacterial and fungal compositions, were determined. The variation in edaphic parameters (pH, electrical conductivity, organic carbon, particle size distribution, etc) through the deterioration transect caused a decrease in plant and microbial diversities, as well as in microbial productivity measured by enzymatic activities. Variation in bacterial relative abundances through the transect was better explained by soil fertility related factors, such as organic matter, pH, salinity, enzymatic activities or microbial biomass (which in turn were related to the presence of plants), than by metals concentrations. The presence of vegetation at the tailings allowed the transition from lithotrophic bacterial orders which dominated in bare tailings areas (e.g. Acidimicrobiales, Chromatiales) to organotrophic oriented orders (e.g. Cytophagales, Actinomycetales). Vegetated patches at the tailings pile and its borders shared some organotrophic bacteria with control forest samples (e.g. Rhizobiales), indicating that natural plant mediated successional processes might stimulate biogeochemical cycles similar to those occurring at non-polluted systems. This functionality of the impacted sites, which include unfavourable edaphic conditions, should be taken into account in the phytomanagement of mine tailings, since it may support its long-term sustainability. Unlike bacteria, variations in fungal relative abundances through the transect were not clearly explained by soil parameters or the presence of plants. Other factors related to spatial distribution or the type of organic substrates may be more determinant in the case of fungi., Competing Interests: Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

This study aimed to assess the effectiveness of biochar from sewage sludge -BSS- and from pruning trees -BPT- (addition dose of 6% d.w.) to immobilise metals in acidic (pH ∼ 4.7) and basic (pH ∼ 7.4) mine soils under variable flooding conditions, and to determine biochar influence on plant (Sarcocornia fruticosa -Sf-) growth and metal uptake. BSS had lower pH (∼8.2 vs. ∼ 9.8), CaCO 3 (∼71 vs. ∼ 85 g kg -1 ), total organic carbon (∼354 vs. ∼ 656 g kg -1 ) and higher water soluble organic carbon (WSOC ∼ 0.15 vs ∼ 0.06 mg kg -1 ) than BPT. PVC columns (15 × 30 cm) were prepared with the following treatments (n = 4): 1) no Biochar-no Sf; 2) no Biochar-Sf; 3) BSS-no Sf; 4) BSS-Sf; 5) BPT-no Sf; 6) BPT-Sf. Changes in water table level (WL) were simulated for 303 days with tap water (upper 0-15 cm alternating flooding-drying conditions, lower 15-30 cm always underwater). The pH, redox potential (Eh), temperature and porewater WSOC, Zn, Cd and Pb concentrations were regularly measured, and plants were removed at the end and length, fresh weight and metal concentrations in tissues measured. In the basic soil, there were no consistent evidences that BSS and BPT were effective decreasing porewater metal concentrations and reducing metal uptake in plants. Sf contributed to mobilise Zn, and in less extent Cd, in the upper soil layer, regardless of the type of biochar, and this effect increased with aging. In the acidic soil, BSS and BPT were effective increasing the pH and decreasing porewater metals. BSS increased its efficiency with aging, which can be mainly attributable to the more reduced conditions that induced (lower Eh values) due to its higher WSOC content. Biochar was effective hindering metal mobilisation by Sf and reducing plant's metal uptake (e.g. reduction in roots: ∼7 fold for Zn, ∼19-fold for Cd and ∼ 2-fold for Pb). BSS was more effective promoting Sf growth (fresh weight) than BPT. Therefore, in relation with the use of biochar from sewage sludge and from pruning trees as soil amendments under flooding-non flooding conditions, we can conclude that it can be a useful option in acidic mine soils for decreasing water soluble metals and improving plant growth. However, in basic mine soils, we have no evidences to support the advantages of using these two types of biochar as amendments. Hence, the use of biochar in metal-polluted wetlands has environmental implications that must be planned for each specific case in order to optimize the positive aspects (wetland as sinks of pollutants) and reduce the drawbacks (wetland as source of pollutants)., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Phytomanagement in terms of phytostabilisation has been proposed as a suitable technique to decrease the environmental risks of metal(loid) enriched mine tailings. Nevertheless, at these sites some issues must be solved to assure the long-term establishment of vegetation (e.g. salinity, low fertility, metal(loid) phytotoxicity, etc.) The objective of this study was to assess the effects of the addition of a municipal solid waste on a mine tailings soil and on the growth and metal(loid) accumulation of a grass plant species (Piptatherum miliaceum). In addition, the effects of intra-specific interactions were evaluated. A pot experiment was performed during 8 months, including two soil treatments: the mine soil and its combination with municipal solid wastes. For each treatment, pots without plants, pots with one plant, and pots with two plants were arranged. The addition of municipal solid wastes improved the soil fertility and plant growth in the mine soil, but also increased the mobile fractions of Zn, Pb, Cd, Mn and Ni. Plants in the amended treatments showed better nutritional status (higher P and K). Stable isotope δ(15)N was associated to the better nutritional status, while δ(13)C and δ(18)O indicated higher photosynthetic efficiency and stomatal conductance in amended treatments. Although the accumulation in leaves of most metal(loid)s decreased with the municipal waste application, the concentrations in both treatments did not exceed toxic limits for fodder. There was an effect of intra-specific competition in plant growth, probably due to lack of nutrients in the mining soil or limited pots volume in the treatments with municipal waste., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The goal of this work was to assess the selection of the most suitable combination of plant species for the phytomanagement of mine tailings, by comparing among different plant life-forms (grasses, shrubs and trees). A comparison on induced rhizosphere changes generated by four plant species (the grass Piptatherum miliaceum, the shrub Helichrysum decumbens, and the trees, Pinus halepensis and Tetraclinis articulata) and high density vegetation patches (fertility islands) at a mine tailing located at Southeast Spain and the description of their physiological status employing stable isotopes analyses were carried out. The edaphic niches for plant growth were determined by salinity, organic matter and total soil nitrogen while metal(loid)s concentrations played a minor role. Induced changes in plant rhizospheres had a significant impact in soil microbiology. While grasses and shrubs may play an important role in primary ecological succession, trees seem to be the key to the development of fertility islands. The low δ(15)N values (-8.00‰) in P. halepensis needles may reflect higher ectomycorrhizal dependence. Large differences in leaf δ(18)O among the plant species indicated contrasting and complementary water acquisition strategies. Leaf δ(13)C values (-27.6‰) suggested that T. articulata had higher water use efficiency than the rest of species (-29.9‰). The implement of a diverse set of plant species with contrasting life forms for revegetating tailings may result in a more efficient employment of water resources and a higher biodiversity not only in relation to flora but soil microbiology too., (Copyright © 2013 Elsevier Ltd. All rights reserved.)