Effects of earthworm (Pheretima sp.) on three sequential ryegrass harvests for remediating lead/zinc mine tailings

Farm soil and artificial soil were mixed separately with Pb/Zn mine tailings and placed in the ratios (w/w) of 0, 12.5, 25, and 50% in pots, together with 0.33 g KH2PO4 and 0.35 g urea/pot. Each pot contained 15 ryegrass seedlings (Lolium multiflorum) and 7 earthworms (Pheretima sp.). At the end of the experiment (after the third harvests of ryegrass), the earthworms were all found to be alive and burrowed completely at the bottom of the pots in all treatments. Earthworm activity increased plant biomass significantly in artificial soil/tailings mixtures in the second and third harvests as compared with the control (without addition of earthworms), but not in the farm soil/tailings mixture. The biomass of ryegrass in both soil/tailings mixtures decreased with increase of tailings, especially those containing 50% tailings. In general, the plant biomass obtained from farm soil/tailings mixtures was lower than that from artificial soil/tailings mixtures. Earthworm activity did not significantly increase DTPA-extracted Zn or Pb, in both soil/tailings mixtures. However, the concentrations in farm soil/tailings mixtures were lower in the second and third harvests, compared with the first harvest. The Zn and Pb uptake by ryegrass shoots in the two soils/tailings mixtures did not increase significantly (compared with the control) in the first and second harvests. However, in the third harvest, Zn and Pb uptake increased significantly in farm soil/tailings mixtures, but decreased significantly in artificial soil/tailings mixtures (compared with the control). The results indicated that the presence of earthworms and sequential plant harvesting could be a viable strategy for the remediation of Pb/Zn tailings.