Long-term effects of application of sewage sludge to soil on composition of herbage with respect to potentially toxic elements

Repeated applications of metal-contaminated sewage sludge can have a drastic effect on soil levels of trace elements and lead to serious toxicity effects in plants. In some cases, land can be rendered sterile.It has been demonstrated that contamination of soils with respect to cadmium, copper, lead, mercury, nickel and zinc is largely irreversible, although there does appear to be a long-term tendency for these metals to become progressively less available to plants over a long period of time. Most national guidelines designed to regulate the disposal of sewage sludge on agricultural land are based on the assumption that relatively rapid fixation of contaminant metals does take place in the soil after sludge application. There is a dearth of information relating to the rates at which potentially toxic-elements commonly present in sewage sludge become immobilised in soils, although it is clear that contaminant boron can be leached down the profile in the short term.Evidence is presented that contamination of top soil can persist for a period of six years after a single application of sludge (150 tonnes dry matter/ha). Over this period, there was little change in available levels of boron, cadmium, copper, lead and zinc in the top soil and the degree of enhancement of these elements in perennial ryegrass grown in the sludge-treated area remained more or less unchanged.