A LONG-TERM STUDY OF THE EFFECTS OF HEAVY METALS IN SEWAGE SLUDGE ON SOIL FERTILITY AND SOIL MICROBIAL ACTIVITY

摘要

The amendment of agricultural soils is generally considered to be the best practical environmental option for utilising the nutrient and organic matter content of sewage sludge, yet there is concern that heavy metals present could build up in the soil and have a long-term negative effect on soil fertility. In 1994 a long-term study was started at nine sites throughout Britain to evaluate the effects of heavy metals, added as a result of sludge application, on soil microbial activity and soil fertility. The sites were chosen to cover a range of soil properties, in particular pH and clay and organic matter contents, and different land uses (arable and grassland). Selected metal-rich sludge cakes were applied over a four year period (1994-97) to establish zinc (Zn), copper (Cu) and cadmium (Cd) metal dose-response curves in excess of current total soil limit values. The effects of Zn, Cu and Cd on soil microbial processes and soil chemical properties are being examined, with particular reference to nitrogen-fixing rhizobia, soil microbial respiration and biomass carbon, and soil extractable metal concentrations.Ammonium nitrate extractable soil metal concentrations on the individual metal-rich sludge treatments were up to 53 mg Zn kg~(-1), 3.8 mg Cu kg~(-1) and 0.18 mg Cd kg~(-1), respectively, at the end of the four year sludge application cycle. Soil microbial biomass carbon contents and respiration rates increased on most of the sludge treatments compared with the untreated control. Rhizobia presence was confirmed at all sites after the four years of sludge cake application. Rhizobium most probable number (MPN) counts showed no treatment differences at seven of the nine sites, although at two of the sites they were lower (P <0.05) on the elevated Zn treatments than on the untreated control.