RESUMO
Soil microbial processes are readily disturbed by added zinc (Zn) in laboratory ecotoxicity tests. This study compares Zn toxicity between freshly spiked soils and soils that have been contaminated with Zn in the field. Soils were sampled in three transects (< 80 m) toward galvanized electricity transmission towers (pylons). The soil total Zn concentrations gradually increased in each transect from background values (25-82 mg Zn/kg) to elevated Zn concentrations near the pylon (226-595 mg Zn/kg). Soil samples taken at the furthest distance from the Zn source were spiked with ZnCl2 to a range of total Zn concentrations similar to those in the transect. Nitrification, respiration, and N-mineralization rates were significantly reduced by added Zn in laboratory-spiked soils and were 9 to 95% (mean 32%) of the control values at largest doses depending on soil type and the microbial process. In contrast, these processes were either unaffected by soil Zn (p > 0.05) or increased significantly with soil Zn concentrations in the transect soils. These increases could not be explained by soil pH or % soil organic carbon. Leaching soils after spiking significantly lowered the toxic effects of Zn on nitrification or on substrate-induced respiration. The soil solution Zn concentrations of field soils were always smaller than in spiked soils at equivalent total Zn. Highest soil solution Zn concentrations were always lower than the soil-solution EC50s of spiked soils. It is concluded that there is a large discrepancy in microbial responses to elevated Zn between spiked soils (unleached) and field-contaminated soils and there is a need to explain this discrepancy in terms of Zn availability, adaptation processes, and additional soil factors controlling the microbial processes.