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Afforestation on contaminated soils contributes to stabilization of trace elements, and to carbon accumulation in biomass and soil. We have assessed two ecosystem services - phytoremediation and carbon sequestration – delivered by trees and shrubs planted on a contaminated land.

The Guadiamar Green Corridor (SW Spain) is a large-scale case of phytoremediation. After the 1998 mine spill, which contaminated over 4000 ha, soil was cleaned up, remediated and afforested. For phytoremediation assessment, we selected seven tree species and measured concentration of 24 mineral elements, 13C and 15N, in five ecosystem compartments: leaves, roots, litterfall, topsoil and deep soil. For carbon sequestration assessment, we selected four woody species in three sites. We measured soil carbon fractions, microbial biomass and a suit of soil enzyme activities. Soil respiration was monitored over one year in one of the sites.

Tree leaves had the highest variation in chemical concentration (mean CV 79%), and the identity of tree species was a major source of variation. In contrast, topsoil had lower variation (mean CV 25%) and only three elements presented significant differences associated to tree species. Microbial biomass and activity was strongly influenced by soil pH, increasing under trees which neutralized soil acidity, like Populus. Soil underneath trees accumulated more organic matter and lost less C by respiration than in the open microsites.

Soil chemical footprint of seven planted tree species was still weak, 16 years after plantation, probably due to their young age. However, those trees …