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Institut für Landschafts- und Pflanzenökologie

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    Ecology of alluvial arable land polluted by copper mine tailings : new insights for restoration
    (2013) Nikolic, Nina; Böcker, Reinhard
    Mining and extraction of metals generates huge amounts of tailings waste (a mixture of water, finely ground ore rock and processing effluents, which remains after removal of mineral concentrate). Unfortunately, accidental release of mine tailings into river systems and their further deposition in floodplains, often over arable land, has been reported from many parts of the world, with environmental implications difficult to predict. Mine tailings from sulphidic metal ores combine high potential for generating soil acidity, and high concentrations of plant available heavy metals, which are the two most intractable issues in restoration of post-mining sites. On the other hand, barren land degraded by mining waste provides an exciting opportunity to reveal some important ecological principles which might not be apparent under ?normal? conditions. Understanding of the process of primary vegetation succession is in particular crucial for the practice of ecological restoration, which is increasingly seen as a preferable alternative to technical reclamation of land degraded by mining. This work addresses the two major issues relevant for ecology and restoration of alluvial arable land polluted by mining waste, which have so far been very poorly understood: Firstly, the long-term fate of contaminants and their actual impact on soil quality and crop production under true field conditions. Secondly, the process of spontaneous revegetation of barren land under the extreme soil conditions and the environmental setup considerably different from the well-studied Central European. The explorative study was undertaken on an exceptional locality created by long-term and large scale-fluvial deposition of sulphidic copper (Cu) tailings over alluvial fields in Eastern Serbia. Comprehensive surveys of spontaneous vegetation, weed assemblages and cereal crops (species cover-abundance; biomass per m2; and foliar mineral analyses), and concomitant surveys of rhizosphere soils (31 physical and chemical parameter) included 297 sampling locations throughout the polluted floodplain (flexible sampling scheme based on visual appearance of vegetation). Data were jointly analysed in a gradient approach framework by different multivariate statistical methods (ordination: NMS, PCA, CCA; classification: agglomerative clustering; group comparisons: MRPP, ISA; habitat modelling: NPMR; and, regression analysis). The results revealed exceptionally strong structure in the vegetation which was highly correlated with the measured soil properties; the regular change of vegetation and soil properties occurred along spatially explicit transects perpendicular to the river channel. The clear gradients observed in this ?field laboratory? research brought forward some new ecological patterns which had not been reported by other relevant studies so far: 1. Severe P deficiency, most likely not amenable by fertilization, is identified as the master limiting soil factor for grains under the multiple abiotic stresses caused by deposition of sulphidic Cu mining waste. Other plausible soil constraints (low pH, excessive concentrations of available Cu and Al, deficiency of N and B; P deficiency-induced increase of As uptake; competition with superior hemicryptophytic weeds), have remained in domain of a ?black-box?. 2. Frequent ploughing accelerates the substitution of high plant available Cu by nutrient deficiency (primarily P and microelements) and excessive exchangeable Al. Thus, agronomic soil quality indicated by crop yield does not improve even when Cu is leached to the background levels. The environmental consequences of intensive land use are however drastic, and increase vulnerability of these soils to further environmental hazards. On the other hand, one type of spontaneously occurring pioneer forests was shown to considerably slow down the oxidative weathering of the tailings deposits and thus prevent Cu leaching via phytostabilization. 3. Although many characteristics of cereal weed assemblages markedly change along the soil pollution gradient, the species? ability to maintain leaf P homeostasis appears to be the key adaptation underlying the observed vegetation structure. The novel associations of unusual species combinations showed a set of functional adaptations which have enabled the leaf N:P homeostasis and thus the uninterrupted survival of weed vegetation along the soil gradient. 4. The proposed conceptual model describes a highly patterned process of spontaneous revegetation of the created barren land under the severe environmental filtering. In this process, the xerothermic surrounding vegetation adapted to calcareous soils has a very limited role. Instead, primary succession relies on novel types of early vegetation which comprise not only novel combinations of species, but also the key role of species which are novel to the affected region, and do not survive outside of the polluted area. We demonstrate for the first time that pollution-induced severe nutrient deficiency can override the well-established importance of both surrounding vegetation and water level gradient for primary succession. Overall, this study implies that severe nutrient deficiency, which is often a neglected issue in studies of metal-polluted sites, can override the effects of Cu toxicity, the role of surrounding vegetation, and even the role of water level gradients, on the process of spontaneous restoration of vegetation cover. It is highly unlikely that spontaneous restoration under the current conditions would allow the reestablishment of the original (or close to original) alluvial vegetation or pre-pollution agricultural land use. Weathering of the substantial residues of sulphides, further acidification and Cu leaching is the major environmental risk; ploughing (or any mechanical soil disturbance with similar effect) of this vast marginalized area should be avoided. Maintenance of the natural flooding regime, which had, bizarrely, caused this vast degradation in the first place, is indicated important for the current spontaneous succession and warrants further research.