Effect of sewage sludge on mineralisation of organic carbon in soil

Abstract

SOME 40% of the sludge produced at inland sewage works in England and Wales is applied to agricultural land¹. In addition to the plant nutrients and organic matter they provide, such sludges also contain various quantities of heavy metals². In some cases, repeated and very heavy dressings of sludge have led to accumulations of harmful elements that are toxic to plants³. There has been concern about their possible accumulation in agricultural soils receiving sludge⁴ since, as Tyler⁵ suggested, microbial activity in soils is susceptible to enhanced levels of heavy metals. The state of soils receiving sludge is monitored by regular analyses⁶, and the Advisory Services of the Ministry of Agriculture (ADAS) have recommended a combined upper limit⁷ to the total load (pre-existing and applied) of Zn, Cu and Ni, above which crop plants may be affected. We have investigated whether the approved quantities of sludge have adverse effects on the microorganisms responsible, in aerobic conditions, for mineralising the organic carbon in the soil, and have shown that three contrasting sludges from the same works, when mixed with soil in amounts up to five times the recommended limit, had no adverse effect on carbon mineralisation.