Where's the limit? Changes in the microbiological properties of agricultural soils at low levels of metal contamination

A number of microbial properties previously shown to be sensitive to heavy metal toxicity were determined in soils from field experiments at Brunnby and Robacksdalen in Sweden. The properties investigated were the acetylene reduction activity (ARA) potential of blue-green algae and heterotrophic soil bacteria, population size of Rhizobium leguminosarum bv. trifolii, size of the microbial biomass, basal respiration, specific respiration rate of the biomass, bacterial community metal tolerance, phospholipid fatty acid pattern of the soil microbial community and lag period and specific microbial growth rate determined after glucose addition to soil. In the Brunnby soils, sewage sludge applications between 1966 and 1989 had increased the soil C content from 2.3 to 2.6% and reduced soil pH from 6.1 to 5.8. Concentrations of Cd, Cr, Cu, Pb and Zn in the soil had increased by up to 76%, but had not reached the current lower EC limits for soils. Most of the measured microbial properties were affected by the sludge additions, although effects were generally moderate. We observed reductions of between 15 and 80% in autotrophic and heterotrophic ARA potential, in numbers of rhizobia and in the biomass C-to-organic C ratio, and increases between 25 and 76% in the specific microbial respiration rate, and in the lag time and the specific microbial growth rate upon glucose addition. There were significant differences in the community structure determined by phospholipid fatty acid (PLFA) patterns between the high sludge treatment and the control and low sludge treatment, and in the bacterial community Cu tolerance between the high sludge treatment and the control. Basal respiration was not significantly affected by past sludge additions. At Robacksdalen, additions of metal salt solutions between 1979 and 1991 had increased soil concentrations of Cd, Cu and Pb by up to 23%, but soil concentrations were nevertheless below the background concentrations at Brunnby. In spite of the low metal concentrations, small, but statistically significant effects of metal addition on the specific respiration rate, lag time before the onset of microbial growth upon glucose addition and on potential autotrophic and heterotrophic ARA were found. The findings are discussed in relation to current legislation for soil protection.