Soil nematode responses to year-to-year variation of low levels of heavy metals

The composition of the nematode assemblage in pasture plots that received sewage sludge amended with 4 rates of Cu, Ni, or Zn was assessed in 5 years; the highest rate of application of each metal was near the New Zealand regulatory limit. The activity of heavy metals in soil solution was determined at each sampling. In terms of management, pasture conditions in 1999 and 2000 were most settled, and while the activities of Cu, Ni, and Zn explained some 44% of variance in total nematodes, there were few significant correlations between nematode genera and individual heavy metals. Both Cu and Zn depressed nematode diversity (H′) and the decomposition index. There were, however, no consistent effects on any nematode functional group. Over the 5 years sampled there were few consistent correlations between soil nematodes and heavy metal activity, the greatest being significant depression of H′ by Zn in 3 years. There was no effect on the total maturity index. Although the few significant effects may reflect the relatively low heavy metal activities, a detailed analysis of heavy metal impact on the nematode assemblage, and thus its contribution to soil processes, would require relating the 6 functional groups to their food resources in the various plots.