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

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 <4) was observed at the most contaminated sites, which resulted in higher solubility of toxic trace elements and lower enzyme activities (five times lower dehydrogenase and β-glucosidase activities, in comparison to neutral soils). The results suggest that improving soil conditions before plantation through amendment application is critical to ensure the improvement of microbial activity in the long-term at such degraded sites.