Nitrification and denitrification activities of zinc-treated soils worked by the earthworm Pheretima sp.

Little information is available on the roles of earthworms in N transformation and biological activities in heavy metal-contaminated soils. The purpose of the present paper is to examine the effects of earthworm on nitrifying and denitrifying enzyme activities of Zn-treated soils. ZnCl2 solution was added to soils at rates ranging from 0 to 400 mg Zn kg(-1) and was allowed to fractionate and stabilize for 60 days. The Zn-tolerant earthworm Pheretima sp., which had been previously screened, was then introduced into Zn-treated soils and allowed to work for 40 days. Original soils without the addition of Zn and introduction of earthworms but incubated under the same conditions were used as controls. Zn-treated soils without the introduction of earthworms were also used as controls. Results showed that earthworm activity increased both nitrifying and denitrifying enzyme activities of the soils. These beneficial effects of earthworms were particularly evident in the treatments where Zn had an adverse effect. Zn added at a rate less than or equal to200 mg Zn kg(-1) generally had a stimulating effect on the enzyme activities measured. However, Zn added at a rate >200 mg Zn kg(-1) had an adverse effect on nitrifying and denitrifying enzyme activities, which could be alleviated or even completely eliminated by earthworm activities.