Solubility and Availability of Heavy Metals in Soils as Affected by Compost Amendment

The effect of agriculture solid waste compost on the availability of native and spiked Cd, Cu, Ni, and Zn in three different soils was evaluated. The three soils were spiked separately with the metals at concentration of 800 mg kg soil-1 and amended with three rates of the compost (0, 3, and 6 %-w/w). The mixtures of soil-compost were allowed to age at room temperature for 30 and 90 days. After each period, they were extracted for readily soluble and exchangeable (2.5% acetic acid), available (DTPA), and bound to organic matter (sodium pyrophosphate) metal species. Addition of compost generally reduced the extractability of the soluble and exchangeable forms of all native and spiked metals. This effect was directly related to the amount of added compost and increased with ageing time. The potentially plant-available Cd and Cu extracted with DTPA were generally reduced with increasing additions of compost and incubation time. There was increase of sodium pyrophosphate extractable Cd and Cu as a result of compost amendment and incubation time increase. Conversely, recovery of Zn in DTPA-extract was larger with increasing both compost and incubation period whereas Ni remained invariant with compost additions with slightly decrease in metal solubility during aging. The obtained results suggest that soil amendment with compost-organic matter may accelerate the immobilization of native and spiked heavy metals in soils due to reducing readily soluble and exchangeable all metals in addition to reducing possible plant uptake of- at least- Cd and Cu.