Shrubs for the remediation of contaminated Mediterranean areas: Is the nurse effect mediated by increases in soil enzyme activities?

•Using shrubs as nurse plants for tree recruitment is useful for restoring highly degraded sites.•The nurse effect is often reported to be mediated by increases in microbial activity underneath shrubs.•13 years after revegetation of a contaminated area, soil fertility and enzyme activities were analyzed in a nurse shrub-tree association.•Enzymes were influenced scarcely by vegetation development, but strongly by pH and organic matter.•In this area, the nurse effect was not mediated by increases in soil microbial activity. The use of shrubs as nurse plants to facilitate woody plant recruitment has been proved to be particularly useful for the revegetation of highly disturbed environments, such as contaminated lands. In a contaminated area from SW Spain, we compared soil fertility and microbial enzyme activity in different microhabitats associated with shrubs, along a gradient of soil contamination, 13 years after the revegetation of the area following a contamination episode. We compared soil organic matter, nutrient content, and enzyme activities in four microhabitats: underneath retama (Retama sphaerocarpa) shrubs nursing Holm oak (Quercus ilex subsp. ballota) seedlings, underneath separate retama and oak individual plants, and in patches of soil without any woody cover. Soil enzyme activities were influenced more by the background soil conditions (pH and organic matter content, modified by the addition of a soil amendment during soil remediation) than by the development of the vegetation. In the soils under the cover of the retama-oak association, intense acidification (pH