Comparative Studies on the Effect of Eisenia foetida and Allolobaphora caliginosa on Bioavailability of Zn and Pb in Three Soils

The effects of two earthworm species (Eisenia foetida and Allolobaphora caliginosa) on the bioavailability of Zn or Pb were studied in three soils (Cinnamon soil, Aquic brown soil and Acid brown soil) spiked with 200 mg kg^-1 Zn or 400 mg kg^-1 Pb, respectively. The results showed that Eisenia foetida and Allolobaphora caliginosa were tolerant to Zn or Pb. but the growth of earthworms was adversely impacted in Zn or Pb contaminated soils. The effects of two earthworm species on pH values were no significant differences. Soil DOC content was significantly enhanced by two earthworm species when compared with control. In Aquic brown soil and Acid brown soil spiked with Zn, the DOC concentrations were enhanced more by Eisenia foetida than by Allolobaphora caliginosa., but in three soils spiked with Pb the DOC content of soil with Eisenia foetida was not significantly different from that of soil with Allolobaphora caliginosa. The concentration of DTPA extractable Zn was increased in Cinnamon soil, but decreased in Aquic brown soil, and unchanged in Acid brown soil by both earthworm species. No significant difference was found for the total concentrations of Zn and Pb in the casts of two earthworm species, but the concentration of DTPA extractable Zn in cast of Allolobaphora caliginosa was much higher than that of Eisenia foetida in Aquic brown soil and Acid brown soil, but the case of DTPA extractable Pb were just the reverse. Allolobaphora caliginosa had relative higher capacity for Zn mobilization, while Eisenia foetida had relative higher capacity for Pb mobilization in soil.