The effect of cadmium on the activity of nitrifying populations in two different grassland soils

The effect of a laboratory addition of 10, 100 and 500 mg Cd kg(dry soil)(-1) on ammonification and nitrification was studied using soil samples of two unpolluted grassland soils. Calcareous and non-calcareous soil were selected for this purpose. Various parameters of nitrifying activity were investigated simultaneously: activity during long-term laboratory incubations in the presence and absence of a substrate, mineralization potentials, and potential activity of both ammonium and nitrite oxidizers during short-term incubations in soil slurries. Cadmium was added as aqueous CdCl2. Additions of both 100 and 500 mg Cd kg(dry soil)(-1) doses significantly lowered the ability of both soils to nitrify 100 mu g added NH4+-N g(dry soil)-1 as a substrate, which was reflected in a decreased rate of nitrate formation (maximum inhibition reached 60% in the calcareous soil and 45% in the non-calcareous soil). Furthermore, these two concentrations of Cd caused an abnormal accumulation of nitrite immediately after incorporation, particularly in the calcareous soil. The addition of 10 mg Cd kg(dry soil)(-1) intensified N-mineralization in both soils, probably as a consequence of a higher concentration of readily metabolized substrate originating from killed bacteria or fungi. An excess of nitrate was then formed as a final step. The harmful effect of cadmium was more pronounced in calcareous soil, probably due to the higher sensitivity of nitrite-oxidizers in these soil samples.