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The study of the rhizospheric microbiome in native plants should be a prerequisite before carrying out the phytomanagement of mine tailings. The goal of this work was to evaluate the rhizospheric microbiome of Piptatherum miliaceum in semiarid mine tailings. A comprehensive edaphic characterization was performed including the description of soil microbial composition in the rhizosphere of P. miliaceum growing at a mine tailings pile and at a control site. Plant nutritional and isotopic compositions were also determined. Neutral pH of the tailings (7.3) determined low metal extractability in 0.01 M CaCl 2 (e.g. < 1 mg/kg for Zn). In spite of the contrasting edaphic fertility conditions of both sites, N (~ 15 g kg -1 ) and P (~ 400 mg kg -1 ) leaf concentrations were similar. The lower δ 15 N at the tailings plants (- 4.50‰) compared to the control (6.42‰) indicated greater efficiency of P. miliaceum for uptaking N under the low fertility conditions of the tailings (0.1% total soil nitrogen). The presence at the tailings of bacterial orders related to the cycling of N, such as Rhizobiales, could have contributed to enhance N acquisition. The lower leaf δ 13 C values at the tailings (- 30.22‰) compared to the control (- 28.47‰) indicated lower water use efficiency of the tailing plants. Some organotrophic bacterial and fungal groups in the tailings' rhizospheres were also found in the control site (e.g. Cytophagales, Sphingobacteriales for bacteria; Hypocreales, Pleosporales for fungi). This may indicate that P. miliaceum is able to shape its own specific microbiome at the tailings independently from the initial microbial composition of the tailings., (© 2022. The Author(s).)

The phytostabilization of mine tailings requires a previous assessment of the effects of soil amendments on metal mobility. The goal of this work was to evaluate the response of metal availability (both labile and potentially available pools) to the addition of two organic amendments (a municipal waste biosolid and a tree biochar), separately and in combination, in a mine tailings substrate. For this purpose, a comprehensive comparison among several single extraction procedures and a sequential extraction procedure was performed. The effects on metals phytotoxicity were assessed through a germination test using seeds of Zygophyllum fabago. When evaluating the effect of the amendments in the labile metal pool, the biochar resulted effective in decreasing metal-extractable concentrations, especially for Cd, Mn and Zn. The treatment with biochar also showed better germination parameters (percentage of germinated seeds and sooner germination) than the rest of the unamended and amended treatments. The use of the municipal organic biosolid increased labile metal concentrations and potentially available metal pools assessed with EDTA and did not contribute to achieve better results of seed germination. Compared to the single biosolid treatment, the combination of biochar/biosolid modulated some labile metal concentrations and showed similar germination parameters to those obtained for the treatment amended only with biochar. This positive effect of biochar in modulating the soluble metal concentrations associated with certain urban/agricultural organic materials supported the suitability of using these combinations in field applications, although a higher rate of biochar application would be recommended to obtain a more beneficial effect., (© 2021. The Author(s), under exclusive licence to Springer Nature B.V.)

The suitable application of phytomanagement by phytostabilisation using plant tree species in metal-polluted soils requires an assessment of the fate of metals in biological tree compartments. The goal of this work was to evaluate the effect of an urban compost amendment on metal allocation in two evergreen tree species (Pinus halepensis and Tetraclinis articulata) growing in a metal-enriched polluted substrate. A comprehensive characterisation of edaphic parameters and metal speciation was carried out. Plant analyses included metal concentrations in different tree compartments: roots, stems, branches and leaves. The amendment caused a significant increase in plant biomass for both trees, although T. articulata produced 2.5 times more biomass than P. halepensis. The amendment alleviated P deficiency in P. halepensis. This did not occur for the N deficiency detected in T. articulata. The latter showed no effect of the amendment in the allocation of metals, being most of them restricted at the root compartment (> 50%). For P. halepensis, similar behaviour occurred for Cu, Pb and Zn. However, for Cd, the amendment caused its redistribution into pine shoots, probably due to its transport associated with the increased transpiration. Results indicated that T. articulata may be a promising tree species to be used in phytomanagement programs under semiarid climates due to its low metal translocation into shoots and remarkable biomass production under amendment conditions.