Availability of Cd, Ni and Zn to ryegrass in sewage sludge-treated soils at different temperatures

A pot experiment to compare the availability of Cd, Ni and Zn to ryegrass (Lolium perenne L.) was conducted at 15 and 25 degreesC. For this purpose, three rates of sewage sludge (0, 10 and 50 t ha(-1)) were applied in a foamy sand (LS) and a clay loam (CL). Heavy metal availability assessed by soil extractions with 0.05 M CaCl2 and the: organic matter content were monitored during a period of two years, while uptake by ryegrass was monitored over one year after addition of the sludge. The concentrations of Cd and Ni in both the ryegrass and the soil extracts increased significantly, during the first year, especially at 50 t ha(-1). However, in the second year metal availability reached a plateau. During the first year, in the ryegrass Zn concentrations did not show an increase, but in the soil CaCl2-extracted Zn increased. During the same period, the organic matter content decreased rapidly, especially at 25 degreesC, in the first year and much more slowly in the second, giving a total decrease of 16%. Temperature had a marked effect on metal availability; both soil extracts and plant samples from the 25 degreesC treatment had greater concentrations of Cd, Ni and Zn than those at 15 degreesC. This may be attributed to the organic matter, which decomposed more rapidly at 25 degreesC. Moreover, soil-plant transfer coefficients (Tc) of the metals were significantly higher at 25 degreesC than at 15 degreesC, with Cd showing the greatest difference, followed in decreasing order by Zn and Ni.