Sewage sludge (biosolids) contains larger concentrations of heavy metals than most soils and following sludge application to agricultural land may accumulate in the topsoil, as they are not easily leached and crop offtakes are small. The maximum permitted metal concentration limits in UK soils are largely based on metal impacts on crops and human dietary intakes, with no account explicitly made in the regulations of potential impacts on soil micro-organisms. Moreover, an Independent Scientific Committee reviewing the Soil Fertility Aspects of Sludge Applications to Land in the UK recommended that “further research was needed to examine the effects of heavy metals from sewage sludge on soil micro-organisms”.

In 1994, a long-term study was established at nine sites throughout Britain to evaluate the effects of heavy metal additions in sewage sludge on soil microbial activity and long-term soil fertility. The sites were chosen to encompass a range of soil properties (principally clay and organic matter contents) and land uses (arable and grassland). Over a four year period (1994-97), selected metal rich sludge cakes were applied to establish individual zinc (Zn), copper (Cu) and cadmium (Cd) metal dose-response curves up to and exceeding the current soil limit values in the UK (300 mg Zn/kg and 135 mg Cu/kg for soils in the pH range 6-7, and 3 mg Cd/kg).

During phase II (1998-2002) of this long term study the effects of the established heavy metal concentrations on soil microbial processes and chemical properties are being examined, with particular reference to nitrogen fixing rhizobia, soil microbial respiration, biomass carbon and soil extractable metal concentrations. The results from phases I (1994-1998) and II (1998-2002) of this study will be discussed.