Effect of heavy metal contaminated sewage sludge on biological and chemical properties of coniferous forest soils
A field study was carried out at Ardross forest, northern Scotland where heavy metal contaminated sewage sludge had been applied to a peaty podzol at rates of 500 (low) and 1000 (high) kg N ha-1 before tree (Sitka spruce) planting. Nitrogen mineralisation rates determined by field incubation of sealed cores ranged from 3.7 to 4.5 and 7.3 to 9.4 kg N h-1 over the growing season (May to September, 1991) in soils amended with low and high rates of sludge respectively. For the control soil, to which no sludge had been added, mineralisation rates ranged from 2.4 to 2.9 kg N ha-1. Mineralisation of residual sludge was estimated to be 0.56&'37 and 1.14&'37 in 1991, 8 years after sludge application at the low and high rates, respectively. Soils brought back to the laboratory and repacked according to the field profile enabled microcosm studies to be carried out to further investigate possible changes caused to N-cycling processes in coniferous forest soil due to application of heavy metal contaminated sewage sludge, and to consider possible mechanisms of any such changes. In the microcosm study, the two rates of application of sewage sludge increased N mineralisation. A linear relationship was apparent between N mineralisation and the rate of sludge application. Increased N mineralisation was associated with an increase in active fungal mycelium, biomass N and soil animal population densities. The availability of Cd, Cu, Mn, Pb and Zn was found to be related to the time of the year, with highest availability in Spring and Summer, and lowest availability in Winter. There was no evidence of any adverse effects in terms of heavy metals on the studied biological parameters and mineralisation rates in the field and microcosm studies. Total N, pH and moisture also increased due to sludge application.