Effect of Heavy Metals on the Denitrification Process in Soil

The effect of Cd, Cu, Pb, and Zn on denitrification of three Pseudomonas species in a liquid culture medium (Giltay) and in autoclaved soil, as well as on the denitrifying activity of native soil, was investigated. In Giltay medium, Cd in concentrations starting at 50 µ g/ml strongly inhibited growth and denitrification of an unidentified Pseudomonas sp., P. aeruginosa , and P. denitrificans , while Zn and Pb did not influence the denitrifying activity even up to 500 µ g/ml. Cu at 50 µ g/ml inhibited denitrification of P. aeruginosa and the unidentified Pseudomonas sp.; however, it had no influence on P. denitrificans . When the three bacteria were inoculated into autoclaved soil their reaction to Cd and Cu was similar to that in Giltay medium, but the accumulation of nitrite was considerably higher. Zn, which showed no effect in the liquid medium at 500 µ g/ml, inhibited the denitrifying activity of all bacteria in the soil at this concentration. Denitrification in native (nonautoclaved) soil was also inhibited by the addition of increasing amounts of heavy metals. This was indicated by an accumulation of nitrite and nitrous oxide which appeared to a lesser degree and for a shorter time period in nontreated samples.