Influence of perennial ryegrass on a copper and zinc affected terrestrial nematode community
Introduction
Potential risks of pollutants have often been assessed by single species toxicity tests in the laboratory in combination with pollutant concentrations measured in the field. Apart from the difficulty of measuring relevant concentrations of pollutants in the field, discrepancies between results of laboratory and field studies may also be caused by interactions among organisms and between organisms and their environment (Cairns, 1983). In contrast to direct effects due to the action of a toxicant on receptor sites within the organism itself, these indirect effects occur when the pollutant interferes with, for example the food availability or predator-prey interactions. It has been suggested that on the community or ecosystem level, indirect effects gain importance, although they are difficult to predict (Yodzis, 1988).
One way to increase the ecological complexity for the risk assessment of pollutants is to expose indigenous nematode communities in micro- or mesocosms (Parmelee et al., 1993; Korthals et al., 1996a). However, among soil ecotoxicologists these tests are typically done without plants which have a pronounced impact on many biotic and abiotic processes in the soil. Plants may also affect the bioavailability of pollutants, as observed earlier for aquatic ecosystems (Brock et al., 1992), and the toxic effects on plants may in turn influence the nematode community indirectly.
In order to investigate the possible influence of plants on the impact of pollutants on soil organisms, we compared nematode communities after a one year exposure to Cu and Zn added to soil grown with and without L. perenne. This species is the most widely utilized in West European grasslands and is not very sensitive to heavy metals (Dijkshoorn et al., 1979). It is hypothesized that changes in the nematode community structure will depend not only on the metal concentrations in soil, but also on the presence of vegetation.
Section snippets
Soil and treatments
In October 1992, soil was collected from the top 10 cm of an arable field of sandy soil located 3 km NNE of Wageningen, the Netherlands. From 1980 onwards the field had been cropped with silage maize, starch potatoes and oats in a 3-year rotation (silage maize in 1992). Some soil characteristics are listed in Table 1 (Korthals et al., 1996a). For more details on site and soil see Korthals et al. (1996b).
After removing stones and organic material of >1 cm, the fresh soil was mixed and dried to a
Soil analysis
Table 2 shows the results of soil analysis at the termination of the experiment. Over all on an average Cu and Zn treatments L. perenne increased pH by 0.12 (±0.02) from 4.24–4.36, obviously as a consequence of nitrogen being supplied and taken up as nitrate (Dijkshoorn et al., 1983). L. perenne tended to decrease the Cu concentration, but had no consistent effect on the Zn concentration.
Effects on grass growth and metal content
The effects of metal treatments on grass growth and metal content of L. perenne are summarised in Fig. 1Fig.
Ecological effects
Based on the high number of bacterivorous nematodes with a c-p value 1, observed at the start and the significant decline in both control soils at the termination of the experiment we assume that initially the soil was disturbed, probably because the soil was collected shortly after harvest. During the one year duration of the experiment, we assume that the nematode communities followed trends previously observed after disturbance and subsequent recovery (Freckman and Ettema, 1993; Ettema and
Conclusion
The present study demonstrated that a field collected nematode community changes due to biotic (presence of L. perenne) as well as abiotic (heavy metals) factors. The presence of vegetation is a very important factor in determining the ecotoxicological effects of Cu and Zn. In soils with L. perenne the effects of Cu and Zn only occurred at higher metal concentrations, were less severe and were more often caused in an indirect way. The effects of pollution on nematode communities in soil with a
Acknowledgements
The authors like to thank H.H.B. van Megen, H. de Ruiter, T. de Wild-Wessels and W. Menkveld for technical assistance and L. Brussaard, T. Bongers and T. Dueck for valuable comments on the manuscript.
References (29)
- et al.
Assessing nematode communities in agroecosystems of varying human intervention
Agr. Ecosyst. Environ.
(1993) - et al.
Decomposition, nutrient loss and microarthropod densities in herbicide-treated grass litter in a Georgia piedmont agroecosystem
Soil Biol. Biochem.
(1985) Nematode populations in relation to soil environmental factors: a review
Pedobiologia
(1981)How plants affects nematodes
Adv. in Ecol. Res.
(1987)- et al.
Interaction of toxicants and communities: the role of nutrients
Environ. Toxicol. Chem.
(1994) - Bongers, T., 1988. De Nematoden van Nederland., Pirola,...
The Maturity Index: an ecological measure of environmental disturbance based on nematode species composition
Oecologia
(1990)- et al.
Proposed changes of c-p classification for nematodes
Russ. J. Nematol.
(1995) - et al.
Fate and effects of the insecticide Dursbanr 4E in indoor Elodea-dominated and macrophyte-free freshwater model ecosystems: II. Secondary effects on community structure
Arch. Environ. Contam. Toxicol.
(1992) - Cairns, J. Jr., 1983. Are single species toxicity test alone adequate for estimating environmental hazard?...