Biological attributes of rehabilitated soils contaminated with heavy metals

This study aimed to evaluate the effects of two rehabilitation systems in sites contaminated by Zn, Cu, Pb, and Cd on biological soil attributes [microbial biomass carbon (Cmic), basal and induced respiration, enzymatic activities, microorganism plate count, and bacterial and fungal community diversity and structure by denaturing gradient gel electrophoresis (DGGE)]. These systems (S-1 and S-2) consisted of excavation (trenching) and replacement of contaminated soil by uncontaminated soil in rows with Eucalyptus camaldulensis planting (S-1-R and S-2-R), free of understory vegetation (S-1-BR), or completely covered by Brachiaria decumbens (S-2-BR) in between rows. A contaminated, non-rehabilitated (NR) site and two contamination-free sites [Cerrado (C) and pasture (P)] were used as controls. Cmic, densities of bacteria and actinobacteria, and enzymatic activities (beta-glucosidase, acid phosphatase, and urease) were significantly higher in the rehabilitated sites of system 2 (S-2-R and S-2-BR). However, even under high heavy metal contents (S-1-R), the rehabilitation with eucalyptus was also effective. DGGE analysis revealed similarity in the diversity and structure of bacteria and fungi communities between rehabilitated sites and C site (uncontaminated). Principal component analysis showed clustering of rehabilitated sites (S-2-R and S-2-BR) with contamination-free sites, and S-1-R was intermediate between the most and least contaminated sites, demonstrating that the soil replacement and revegetation improved the biological condition of the soil. The attributes that most explained these clustering were bacterial density, acid phosphatase, beta-glucosidase, fungal and actinobacterial densities, Cmic, and induced respiration.