The importance of edaphic niches and spontaneous vegetation for the phytomanagement of mine tailings under semiarid climate

Resumen

Mine tailings are considered the main responsible of the environmental problems associated to metal mining activities due to the spread of contaminants by water and wind erosion from their bare surfaces. These issues are especially critical when the tailings ponds are located in the proximities of sensible areas such as protected natural sites, urban, agricultural or recreational areas. Phytomanagement in terms of phytostabilisation (the use of plants to inmobilise metal(loid)s within the rhizosphere) might be a suitable cost-effective option for the restoration of mine tailings. However, the success of its application still needs a better understanding of the edaphic, ecological and physiological factors involved in the establishment of vegetation. The present PhD Thesis was focused on the study of the edaphic and ecophysiological factors involved in the spontaneous plant colonisation of mine tailings with the aim to obtain recommendations for the successful phytomanagement of these environments, in a semiarid climate context in the Cartagena-La Union Mining District (Southeast Spain). The specific objectives were: 1. To describe the edaphic gradients and their associated spontaneous vegetation and to identify potentially plant-favourable edaphic niches in abandoned neutral-pH mine tailings located in the Cartagena-La Union Mining District. 2. To assess the suitability of using spontaneous plant species in restoration/stabilisation of mine tailings, focusing on the role of rhizosphere in the improvement of soil conditions, the study of different life forms and the ecophysiological responses to mine tailings environment. 3. To assess the behaviour of tree species on mine tailings to estimate its possibilities as candidates for phytostabilisation and their potential role in the restoration of such environments. 4. To evaluate the response of a spontaneous grass plant species commonly found on mine wastes (Piptatherum miliaceum) growing under controlled conditions in an organic amended mine tailings by employing stable isotopes and considering intra-specific interactions. To achieve these objectives, a field work stage and a pot experiment were performed. The first field survey was focused on the description of the edaphic and ecological gradients along a transect from a non-polluted control site to a mine tailings area. The edaphic patch distribution was found to be determinant for the selective establishment of spontaneous vegetation growing at the tailings. Soil salinity was the main factor involved in the shifts of the plant species inventory and the decrease of plant diversity and richness from the non-polluted area to the tailings plateau, while metal(loid)s soil concentrations played a minor role. The colonisation of the mine tailings by native late successional plant species was related with the fertility gradient generated by the enhancement of soil conditions as a result of the early development of pioneer vegetation. This natural process has provided the occurrence of high dense vegetated patches in the tailings, here called ¿fertility islands¿, which contain climax vegetation of the local area. The usefulness of the spontaneous vegetation for the phytomanagement of mine tailings was firstly assessed through a field study focused on the characterisation of the rhizospheres and the ecophysiology of different plant species (grasses, shrubs and trees). The rhizosphere of spontaneous grasses and shrubs (P. miliaceum, Helichrysum decumbens) showed a significant enhancement of the soil microbiology compared with the bare soil. Moreover, the similarity between the rhizosphere of individual trees (Tetraclinis articulata, and especially Pinus halepensis) and that of the ¿fertility islands¿ may lead to suggest that trees are the key for the development of these dense vegetation areas. In relation to grasses and shrubs, a field survey was conducted in order to evaluate the different performance of spontaneous halophyte and non-halophyte species and to assess if halophytes could be more suitable for specific salty sites on the tailings. In the case of tree species, three specific studies focused on P. halepensis were done in order to gain insight into the ability of pine trees to cope with the soil conditions at the tailings. The first study was focused on the accumulation of metal(loid)s in P. halepensis along a polluted gradient. The second and the third studies dealt with the ecophysiological status, elemental and isotopic foliar composition and internal nutrients and metal(loid)s retranslocation in spontaneous populations of P. halepensis growing on mine tailings. Pine trees showed a good tolerance to the poor soil conditions of the tailings. In spite of a severe foliar P deficiency, pine trees showed substantial macronutrient retranslocation from senescent leaves, which may help them to cope with some nutritional de¿ciencies, especially in relation to K, Mg and P. The analysis of the foliar isotope composition showed that pines growing at mine tailings are less water stressed that those ones from a natural forest in the area (control), and that mycorrhizal association might be critical for the development of these trees on the mine tailings. Moreover, pine trees showed a relatively low metal(loid) uptake and some mechanisms to cope with them such as the immobilisation in less biologically active tissues (Cd, Cu, Pb and Sb in woody stems) or the accumulation in deciduous parts (As, Cd, Sb, Pb and Zn in litter). Finally, a pot experiment was performed using a grass species commonly found on tailings (P. miliaceum) and a mine tailings soil amended with a municipal solid waste obtained from the recycling of metallic containers. P. miliaceum was able to growth on tailings with and without amendment. The addition of the municipal solid waste improved some soil fertility indicators, although the principal effect was the enhancement of the growth and development of the plants. Metal(loid) concentrations in leaves were below the toxic thresholds for fodder in all treatments. Plant growth was also influenced by intra-specific competition, which provoked a decrease of plant biomass in both with and without amendment treatments. Therefore, the conclusions of the Thesis were: 1) The identification of the most favourable/suitable niches for plant growth should be taken into account when phytostabilisation works are carried out on mine tailings. Ploughing or the addition of certain amendments that could raise soil salinity should be carefully evaluated in order to not destroy those favourable niches. 2) The use of combinations of plant species with different life forms and complementary water and nutrient acquisition strategies may result in a more efficient employment of edaphic resources. This may favour the achievement of a long-term stable plant community and may provide a higher resilience to environmental disturbances such as long drought periods typical from semiarid environments. 3) The employment of halophyte species (e.g. Limonium cossonianum, Atriplex halimus, Zygophyllum fabago) may result a suitable option to revegetate and ameliorate the most saline edaphic niches at the tailings due to their suitable adaptation to salinity and higher enhancement of soil properties. 4) The tree species P. halepensis may result a suitable candidate for the phytostabilisation of mine tailings due to its adaptation to semiarid climate conditions and the potential role for facilitating the occurrence of fertility islands. However, it is recommendable the addition of P fertilizers in order to alleviate the strong deficiency on this element at the tailings. 5) The grass species P. miliaceum may be considered in phytostabilisation of mine tailings, but to obtain a faster growth and development, organic amendments should be added. http://repositorio.bib.upct.es/dspace/