Evaluation of the rhizospheric microbiome of the native colonizer Piptatherum miliaceum in semiarid mine tailings.

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 CaCl2 (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 δ15N 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 δ13C 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.

Usefulness of pioneer vegetation for the phytomanagement of metal(loid)s enriched tailings: grasses vs. shrubs vs. trees.

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.

The importance of edaphic niches and pioneer plant species succession for the phytomanagement of mine tailings.

Phytomanagement in terms of phytostabilisation is considered a suitable method to decrease environmental risks of metal(loid) enriched mine tailings. The goal of this study was to identify plant-favourable edaphic niches in mine tailings from a semiarid area, in order to obtain relevant information for further phytostabilisation procedures. For this purpose, a transect-designed sampling from non-disturbed soils to two mine tailings was performed, including the description of soil and plant ecology gradients. Plant ecological indicators showed several stages in plant succession: from weeds to stable patches of late successional plant species. PCA results revealed that plant distribution at the tailings was driven mainly by salinity while metal(loid) concentrations played a minor role. The presence of soil desiccation cracks generated low salinity patches which facilitated favourable niches for plant establishment. Edaphic-patch distribution may condition phytostabilisation since ploughing or the employment of certain amendments should take into account favourable niches for plant growth.

The Mar Menor lagoon (SE Spain): A singular natural ecosystem threatened by human activities.

The Mar Menor lagoon is one of the most important ecological singularities in the Mediterranean area. At the same time, it is an area where many economic and industrial activities meet. The sum of the impacts of mining, agriculture and urban development in the surroundings to the lagoon during the last decades has affected its ecosystem. In this paper, we have reviewed the studies done by researchers over two decades regarding the impacts of human activities in the lagoon, reporting data of contamination levels and the effects generated in the ecosystem of the lagoon. The discharge of enriched nutrient waters seems to be the most important input in the lagoon. Changes to more friendly agricultural techniques in the surrounding areas are therefore necessary. After our review, we can conclude that it is necessary to have higher efforts from the public administrations and stricter environmental regulations in order to preserve, on one hand, the important ecological values of the lagoon, and on other hand, public health.