Aznalcóllar mine spill restoration: Long-term effects of trace element contamination and tree species on soil microbial biomass and enzyme activities

Comunicación Oral en la Conferencia Mine Closure 2018 celebrada en Leipzig (Alemania), dentro de la sesión "Biodiversity and ecological aspects".

Soil microorganisms have a huge influence in the organic matter oxidation, hydrolysis and degradation. Therefore, they are essential to constitute the optimal conditions for the recovery of the ecosystem functions and the development of a vegetation cover in contaminated soils. Restoration measures were carried out after the accidental spill of the Aznalcóllar mine in 1998, where acidic waters and pyrite sludge with high concentration of trace elements (As, Cd, Cu, Pb, S and Zn) were released into the Guadiamar River basin, in Spain. First, the sludge was removed and the soil clean-up, followed by amendments addition and afforestation with native tree species. Almost 20 years after the accident, we evaluated the recovery of the soil functionality and the ecosystem services provided. We analysed contaminated and restored soils in two sites with contrasted pH, trace elements availability and nutrient content. In these sites, different tree species, white poplar (Populus alba), stone pine (Pinus pinea) and wild olive (Olea europaea), were planted, each of them with a singular litter quality input into the soil. Adjacent soils under herbaceous cover were also analysed as non-tree control. In general, tree afforestation contributed to improve belowground activity due to a higher trace element stabilisation in the soils underneath, and enhanced soil microbial biomass and enzyme activities related to C, N and P cycles. However, significant differences among tree species were not found in relation to microbial activity. The ratios Cmic:Corg and Nmic:Ntot revealed a disturbance in the mineralization dynamic which may be explained by the effect of trace elements contamination in microbial biomass. In soils with acidic pH and, therefore, higher trace elements availability the ratios were significantly lower than in neutral pH soils.

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