Effect of earthworm on bio-availability of Cu and Cd in soils

Phytoremediation of heavy metals in contaminated soil has been widely accepted as a cost-effective, environmental-friendly technology. Metal-accumulating plants are usually planted to remove metals from soils by concentrating them in the harvestable parts, while the efficiency depends on the amount of aboveground biomass and the bioavailabililty of metals. Though addition of chelating agents has been shown to increase metal bioavailabililty and phytoremediation efficiency, it costs too much and shows environmental risk. ;Beneficial roles of earthworms on soil fertility, nutrient cycling and plant growth have been commonly observed. Earthworms can influence metal mobility in soil through burrowing and casting activity. The question arises whether earthworm activity can actually increase metal bioavailability and hence promote the uptake by plants, or whether earthworms have a positive effect on phytoremediation. The objective of this study was to investigate the effect of earthworm on the bioavailability of Cu and Cd in soils through a pot experiment. ;Two soils, red soil from Jiangxi Province and Orthic aquisols from Jiangsu Province, were used in the pot experiment. Each soil was divided into seven equal parts of 6^0 kg each and amended respectively to contain 0, 100, 200, 400 mg Cu/kg and 5, 10 and 20 mg Cd/kg by adding appropriate concentrations of CuSO-4 and CdCl-2. After two-month incubation, metal-treated soil samples were divided into two groups with 3 pots for each group as replicates (1 kg soil/pot). One group received six earthworms (Pheretima sp.) for each pot, the other group did not contain any earthworms. All earthworms were collected from an uncontaminated soil in Nanjing Agricultural University and had a mean fresh weight of 0^6g. Each pot was added to 0^3333g KH-2PO-4 and 0^3477g Urea as mineral nutrients and received 15 pre-germinated ryegrass seeds (Lolium multiflorum). The pots were put in greenhouse and were incubated for one month. ;After one month, aboveground ryegrass biomass was weighed. Soil pH (2g soil in 10ml 0^01 mol/L CaCl-2) was tested using a Beckman pH meter. Concentrations of Cu and Cd in ryegrass were determined by using atomic absorption spectrophotometey (AAS) after cineration-digestion. Fractions of the metal in soils were also tested using sequential extraction procedures: (1)Water-soluble Cu and Cd: extracted with deionized water, with a water:soil ratio of 10∶1; (2)CaCl-2-extractable Cu and Cd: extracted with 0^01mol/L CaCl-2, with a solution:soil ratio of 10∶1; (3)DTPA-extractable Cu and Cd: extracted with DTPA, pH 7^3, with a solution:soil ratio of 2∶1. Cu and Cd brought into solution were measured by AAS. ;It was found that earthworm activity significantly decreased the pH for the Red soil, a key chemical factor affecting bioavailability of nutrient elements and heavy metals in soil, but had little effect on pH for the Orthic aquisols. Aboveground biomass of the ryegrass increased dramatically from 33% to 96% in the presence of earthworms except for the treatments with 400 mg Cu/kg soil and 10 mg Cd/kg soil. Earthworm activity markedly increased concentrations of DTPA-extractable Cu and water-soluble Cd for the red soil, however, it did not affect three fractions of Cu or Cd for the Orthic aquisols . Total uptake of Cu by the ryegrass was enhanced by earthworm activity in the two soils, while no significant difference was observed in concentration of Cu or Cd in plant tissue between treatments with and without earthworm addition. ;It is very likely that earthworm activity can increase phytoremediation efficiency by improving soil conditions and enhancing plant growth and the uptake of heavy metals by plant. But this effect of earthworm appeared to be metal and soil specific.