It is well known that the earthworms activities can increase the availability of soil nutrients, improve soil structure, and enhance the biomass of plants in uncontaminated soil. Recently, many researchers found that some metal-tolerant earthworms can survive and even change the fractional distribution of heavy metals in contaminated soil. Furthermore, it has been revealed that earthworms are able to increase metal availability, and therefore, to accumulate more metals in plants through their burrowing and casting activities. It is clear that the influence of soil animals is an important factor on phyto-remedation that must be taken into account. We studied some effects of addition of earthworms (Metaphire guillelmi), corn straw, and in combinations of earthworms and corn straw on the growth and Cu uptake by ryegrass in Cu contaminated pot soils. The experiment consisted of four levels of Cu addition (0, 100, 200, 400 mg kg^-1) and four treatments. The treatments were 1. control (CK); 2.straw mulching only (M); 3. earthworm additions to soil only (E); and 4.straw mulching plus earthworm additions (ME). Each treatment had three replicates. 10 seeds of ryegrass (Lolium multiflorum) were sowed in each pot and harvested after 30 days.
After 30 days incubation, all earthworms were alive and the pot soils were burrowed through by earthworms. Results showed that the biomass of earthworm declined with the increase of the dosage of Cu additions. The biomass of earthworm increased significantly in treatment 4 (ME) as compared it with treatment 3 (E). Not only the earthworms could get more food from the straw, but also could counteract some negative effects of Cu on the earthworms. The rates of straw decomposition in ME treatment were increased about 58.11%-77.32%. The earthworm activities increased root biomass of ryegrass significantly, but did not show the effect on plant root growth. On the contrary, straw enhanced roots biomass significantly instead of shoots biomass.
We also found Cu concentration in the plant shoot and the plant root, as well as plant Cu uptake were enhanced by earthworm’s activities and straw mulching. The increased amount by straw mulching was lower than that of earthworms (E) treatment. The treatment of the earthworm-straw mulching combinations enhanced plant Cu concentration, and the amount increased by it was lower than that of the earthworm treatment(E) but higher than that of straw mulching treatment (M). The accumulation factors of copper in the shoots of ryegrass were increased by 31.22%-121.07%,2.12%-61.28% and 2556%-132.64%, respectively, in treatment 3(E), 2(M), and 4(ME). In conclusion, the earthworm activities, straw-mulching and their interactions may have potential roles in elevating phyto-extraction efficiency in low to medium level Cu contaminated soil.