Nematode communities under stress: the long-term effects of heavy metals in soil treated with sewage sludge

The long-term effects of Ni, Zn+Ni, Cu, Zn+Cu and Zn on the nematode communities in soil treated with sewage sludge were studied in an agroecosystem. The soil was a sandy loam with 9% clay, 1.6–4.0% organic matter (OM) and pH 5.8–7.2. Sewage sludge, artificially enriched with four concentrations of each heavy metal or combinations was applied in 1982 and again in 1986 to some of the plots. Ni and Zn+Ni treatments did not have significant effects on the soil nematode assemblages. Some of the Cu, Zn+Cu and Zn treatments had adverse effects, which correlated with the following changes in their structure, decrease in genus/family richness, decrease in maturity indices (MI and MI2–5) of the free-living nematode assemblages, reduction or elimination of the persisters (K-strategists) in favor of colonizers (r-strategists), decrease or elimination of omnivorous nematodes (Order Dorylaimida, Suborder Dorylaimina) and predatory nematodes (Order Dorylaimida, Suborder Mononchina). The increase in plant-feeding nematodes in the Zn treated plots may be due to the negative effects of the heavy metals on some of their antagonists (e.g., mycorrhizal fungi, root-nodule bacteria from Rhizobium spp.) and the elimination of predacious nematodes in these treatments. Some bacterial-feeding nematodes were found to increase, which was related to the indirect long-term effects of the heavy metals (e.g., lower competition and reduced predation pressure). The most common responses of the nematode communities to soil heavy metal pollution were found to be in agreement with the trends expected in stressed ecosystems and communities, predicted by the hypothesis that stress forces ecosystems into an early stage of development.