Ref: 08/SL/01/6 Price: £65
ISBN: 1 84057 473 9
Sewage Sludge Applications to Agricultural Soils.
This interim report summarises the results to date from a long-term study evaluating the effects of sewage sludge applications to agricultural soils on soil quality and fertility at nine sites in Britain. The work involved investigating the effects of established soil heavy metal concentrations (i.e. Zn, Cu and Cd) on soil microbial activity and fertility.
In Phase I of the project, metal dose-response treatments were established using selected sludge cakes that contained elevated metal concentrations (i.e. c.6,000 mg/kg Zn; c.5,000 mg/kg Cu; c.44mg/kg Cd) and were applied over a 4-year period (1994-1997), which provided a high-level of 'stress' to the soil microbial community. The sludge cakes were applied to achieve a range of soil metal concentrations from background levels to above statutory soil metal limit values.
Additionally, at three sites in England, the effects of Zn, Cu and Cd additions in the metal-rich sludge cakes were compared with those in metal-amended liquid sludge applications, to provide a contrast in the form of metals applied and organic matter loading rates. The metal-amended liquid sludge treatments were expected to represent 'worst-case' treatments in terms of likely metal availabilities, akin to a long-term situation where organic matter levels had declined and stabilised after sludge application.
Three key soil micrbial properties were measured, viz.: (i) soil respiration rate, a measurement of soil microbial activity (ii) soil microbial biomass carbon (C), a measurement of the total soil microbial population and (iii) clover rhizobia numbers, a specific species responsible for nitrogen fixation. Significant (P < 0.05) responses in soil microbial properties (i.e. rhizobia numbers and microbial biomass size) and agricultural crop quality (i.e. grain Cd concentrations) were measured. Sewage Sludge Applications to Agricultural Soils.