Long-term changes in the extractability and bioavailability of zinc and cadmium after sludge application

Changes in the extractability and uptake by crops of sludge metals in a long-term field experiment, started in 1942, were measured to assess whether Zn and Cd are either fixed by the sludge/soil constituents or are released as the sludge organic matter (OM) decomposes. Total and 0.1 M CaCl2-extractable concentrations of Zn and Cd in soil and total concentrations in crops were measured on archived crop and soil samples. Extractability of Zn as a proportion of the total ranged from 0.5 to 3% and that of Cd from 4 to 18%, and were higher in sludge-amended than farmyard manure or fertilizer-amended soils. Over a 23-yr period after 1961, when sludge was last applied, the extractability of both metals fluctuated, but neither decreased or increased consistently. The relationships between total soil and crop metal concentrations were linear, with no evidence of a plateau across the range of soil metal concentrations achieved. The slopes of the soil-plant relationships depended on the type of crop or crop part examined, but were generally in the order red beet (Beta vulgaris L.) > sugar beet (Beta vulgaris L.) > carrot (Daucus carota L.) > barley (Hordeum vulgare L.). However, there also were large seasonal differences in metal concentrations in the crops. It is concluded from the available evidence that up to 23 yr after sludge applications cease, Zn and Cd extractability and bioavailability do not decrease.