Recovery of biochemical functionality in polluted flood-plain soils: The role of microhabitat differentiation through revegetation and rehabilitation of the river dynamics

Soil biogeochemical functions in flood-plains are controlled mainly by interactions between river flooding dynamics and vegetation change. This generates a pattern of landscape cross-sectional and longitudinal heterogeneity in texture, microtopography and plant cover. Agricultural uses restrain such mechanisms, eliminating the mosaic of soil environments and vegetation patches in natural flood-plains. The ecological restoration performed in ca. 5000 ha of agricultural lands in the Guadiamar river basin (SW Spain), affected by the Aználcollar mine spill in 1998 (Cd, Cu, Pb and Zn-rich pyritic sludge), has focused on the re-creation of those controlling mechanisms (enhancement of the natural river flooding dynamics and patchy afforestation). We have studied temporal trends, and the role of specific habitats differentiation in the river terraces, on the recovery of the soil biochemical status in the emerging ecosystems. During 2000–2004, the geometric mean of enzyme activities (dehydrogenase, β-glucosidase, urease, arylsulfatase, acid and alkaline phosphatase) increased three-fold in the most-impacted, coarser textured, upper-watershed soils; and by six-fold in less polluted, loamy soils at the mid-watershed. In 2005, sampling was stratified by microhabitats at two representative watershed sections. Vegetation cover-type and transport/sedimentation processes are the main driving forces increasing both the mean value and intra-site spatial heterogeneity of soil properties (especially enzyme activities) in reclaimed areas. In the wet season, soil enzyme activity under adult trees (holm-oaks and Eucaliptus), and in spots where silt and plant residues had accumulated during previous floodings, was more than 50% higher than in bare areas. However, activities were strongly inhibited in eroded areas where pollutant residues appeared in the surface. Woody patches and the grassy matrix of revegetated areas showed distinctive soil N features. Specific plant species effects were observed, such as a generally high enzymatic activity in soils under Tamarix gallica.