G.W. Korthals

Short-term effects of cadmium, copper, nickel and zinc on soil nematodes from different feeding and life-history strategy groups

Abstract The effects of cadmium, copper, nickel and zinc on a nematode community were examined with a ‘natural soil method’. Changes in the indigenous nematode community structure were studied 1–2 weeks after the addition of these metals (as sulphates) to soil collected from an agroecosystem. The soil was acid and only contained a moderate quantity of organic matter as the main metal-binding constituent. As a result, its metal-binding capacity was rather low. The nematode community was found to be affected by increasing concentrations of Cu, Ni and Zn up to 1600 mg kg−1, but not by Cd up to 160 mg kg−1. EC50 values for the reduction in population size of individual taxa showed a low intra-taxon variation for Cu, Ni and Zn. For these heavy metals, uptake and elimination processes as well as their final effect appear similar within the same taxon. Omnivorous and predatory nematodes, known to be K-strategists, were among the most sensitive taxa, and were already significantly affected by 100 mg kg−1 Cu, Ni or Zn added to the soil. The relative abundance of the different life-history groups and, to a lesser extent, the different feeding groups indicated pollution-induced changes in the soil community. However, neither classification predicts the acute effects of Cu, Ni and Zn on different nematode genera in an adequate way.

Inverse relationship between the nematode maturity index and plant parasite index under enriched nutrient conditions.

Abstract The maturity index, an ecological measure of the state of succession, is based on the composition of the nematode fauna. Nematodes that feed on higher plants were originally excluded; a separate plant parasite index was proposed for nematodes feeding on higher plants. In the present study it is shown that, under certain conditions, the maturity index and plant parasite index are inversely related. Therefore it is concluded that the inclusion of plant feeders into the maturity index, as recently suggested, leads to an index which is less sensitive to changes in the environment than the original maturity index. The divergence of maturity index and plant parasite index is confirmed by literature data and is assumed to be a nutrient enrichment effect. The ratio between maturity index and plant parasite index is proposed as a sensitive parameter for monitoring agro-ecosystems.

Organic amendments and their influences on plant-parasitic and free-living nematodes: a promising method for nematode management?

The use of organic soil amendments, such as green manures, animal manures, composts or slurries, certainly has many advantageous aspects for soil quality and is suggested as a promising tool for the management of plant-parasitic nematodes. However, during a recent literature survey we also found numerous studies reporting an increase of plant-parasitic nematodes after the use of organic amendments. Therefore, we critically re-evaluated the usefulness of organic amendments for nematode management and suggest possible mechanisms for a stimulation of plant-parasitic nematodes, as well as mechanisms that might be causing a reduction of plant-parasitic nematodes. In addition, we also elucidate a possible mechanism that might be responsible for the observed overall positive effects of organic amendments on crop yields. It is likely that a significant part of this is, inter alia, due to the proliferation of non-pathogenic, free-living nematodes and their overall positive effects on soil microbial populations, organic matter decomposition, nutrient availability, plant morphology and ecosystem stability.

Chitin amendment increases soil suppressiveness toward plant pathogens and modulates the actinobacterial and oxalobacteraceal communities in an experimental agricultural field.

ABSTRACT A long-term experiment on the effect of chitin addition to soil on the suppression of soilborne pathogens was set up and monitored for 8 years in an experimental field, Vredepeel, The Netherlands. Chitinous matter obtained from shrimps was added to soil top layers on two different occasions, and the suppressiveness of soil toward Verticillium dahliae, as well as plant-pathogenic nematodes, was assessed, in addition to analyses of the abundances and community structures of members of the soil microbiota. The data revealed that chitin amendment had raised the suppressiveness of soil, in particular toward Verticillium dahliae, 9 months after the (second) treatment, extending to 2 years following treatment. Moreover, major effects of the added chitin on the soil microbial communities were detected. First, shifts in both the abundances and structures of the chitin-treated soil microbial communities, both of total soil bacteria and fungi, were found. In addition, the abundances and structures of soil actinobacteria and the Oxalobacteraceae were affected by chitin. At the functional gene level, the abundance of specific (family-18 glycoside hydrolase) chitinase genes carried by the soil bacteria also revealed upshifts as a result of the added chitin. The effects of chitin noted for the Oxalobacteraceae were specifically related to significant upshifts in the abundances of the species Duganella violaceinigra and Massilia plicata. These effects of chitin persisted over the time of the experiment.

Joint Toxicity of Copper and Zinc to a Terrestrial Nematode Community in an Acid Sandy Soil

Heavy metal toxicity to an indigenous nematode community was examined following the addition of Cu and Zn, alone or in combination, to agricultural soil. The dissolved Cu or Zn concentrations measured after equilibrating soil samples with a 0.01 M solution of CaCl2 showed that the metal concentrations found in soils with combined metal additions were not significantly different from those with single metal additions. After an exposure period of six months, many nematode community parameters and individual nematode taxa were significantly affected by increasing concentrations of Cu and Zn up to 200 mg kg−1. Some nematode taxa, such as Thonus, Alaimus and Aporcelaimellus were very sensitive and disappeared at Cu and Zn concentrations exceeding 50 mg kg−1. For several nematode community parameters and nematode taxa, EC50 values for single metal exposures were used to calculate TU50 values for the joint toxicity of Cu and Zn. Based on these calculations, it is concluded that the effects of a combined exposure to Cu and Zn were additive or less than additive. Before this conclusion can be generalised, however, more data are needed on other types of soil, other pH values and other combinations of pollutants.

Tagetes patula as an effective catch crop for long-term control of Pratylenchus penetrans

Tagetes patula is well known to be able to diminish Pratylenchus penetrans (Cobb) populations. In a field experiment, the increase of the P. penetrans soil population density after growing T. patula was significantly less than after chemical soil fumigation with metam sodium. The effect of T. patula on P. penetrans population densities lasted longer than the effect of chemical soil fumigation. Strawberries were grown for 3 consecutive years after T. patula without damage by the root lesion nematode. Assuming a linear rate of increase of the P. penetrans population density, we predict that strawberries could be grown for about 7 successive years without economic damage due to the root lesion nematode. Strawberry yield was greater from the crop grown 3 years after T. patula than that after soil fumigation 4 years previously. About 2 t strawberry yield increase is necessary to allow the growth of Tagetes every 5th year, without economic loss. The possibility of growing both strawberries and Tagetes in the same year should be investigated.

Reaction norms for life-history traits as the basis for the evaluation of critical effect levels of toxicants.

A main objective of ecotoxicology is to assess the effect of chemicals on ecosystems and to evaluate their potential ecological risk (Moriarty, 1983). Consequently, many theoretical and experimental explorations have been made to provide an adequate basis for the effect assessment of toxicants at the ecosystem level. Much attention is paid to the development of statistical methodologies to extrapolate the results obtained from laboratory toxicity tests to the field situation (Van Straalen and Denneman, 1989; Wagner and Lake, 1991; Aldenberg and Slob, 1993). Basically these methods estimate safe environmental concentrations of hazardous compounds from distribution models of critical effect levels obtained from single-species toxicity tests for different organisms

A field study on the host status of different crops for Meloidogyne minor and its damage potential on potatoes.

For several years, a new species of root-knot nematode, Meloidogyne minor, has been reported from parts of The Netherlands, Belgium, UK and Ireland. So far, this species causes most problems on golf courses but has also been reported from a potato field in Zeijerveld (The Netherlands) where it caused strong growth reduction on potato plants, but no damage to potato tubers. As The Netherlands is a potato-producing country, field experiments were set up to evaluate the potential risks this species poses. We tested the host status of some common crops for M. minor under field conditions and, more importantly, also tested its potential to harm potato production in terms of quantity as well as quality. In a 2-year field experiment (2007-2009), the host status of potato (cv. Bartina), rye, annual ryegrass, sugar beet, and maize was tested in the first growing season. Afterwards, these plots were used to evaluate the damage potential of M. minor on two commonly cultivated potato cultivars (cvs Asterix and Markies). In general, only potato seemed to be a good host for this nematode species with a Pf∕Pi-ratio about 1.5. Reproduction was observed mostly on roots but also on tubers, which increases the spread of M. minor by seed potatoes. However, there was no reduction in potato production, neither in yield nor in tuber quality. No significant reproduction could be observed on the other plants (Pf∕Pi values close to zero). From these results one might conclude that this nematode will not become a major threat to European agriculture. However, care has to be taken as within additional glasshouse experiments potato tubers were susceptible for damage caused by M. minor. Thus, further studies on the general biology and ecology of M. minor are needed to make a better risk assessment on this new nematode pest.

A strategy in searching for stress tolerance-correlated characteristics in nematodes while accounting for phylogenetic interdependence

Biological indicators are highly relevant for assessing the condition of a soil as they are integrative; they reflect the overall impact of physical, chemical and biological changes. Indigenous soil organisms are preferable to other test organisms because the diversity and condition of indigenous soil organisms reflect both acute and chronic effects of soil disturbances. Nematodes are ubiquitous, speciose, easily extractable and present in extremely high numbers. Given the ever increasing amount of sequence data, DNA barcode-based community analysis will soon be possible and a next step would be to define objective criteria for the ecological grouping of soil nematodes. Here, we present a framework to ascertain which traits are correlated with a tolerance to stress. For this, a field study on the effects of pH and copper on nematode communities was re-analysed. Changes in abundances of individual genera were correlated with a number of potentially stress tolerance-related characteristics. The generalised least squares (GLS) method was used to account for the phylogenetic dependence of the data. Only the relationship between the ability to enter a survival stage and tolerance to copper at pH 6.1 was found to be significant, but the quantity of missing data probably had a negative impact on the analyses. This study did, however, clearly demonstrate the importance of accounting for the effects of phylogenetic dependence in the data. When the phylogeny was taken into account, we observed an average change in P value of 0.196 (and in some cases as much as 0.6) for the correlations of possible stress-related characteristics and Cu or pH tolerance. This research constitutes a proof-of-principle for a transparent method to relate stress tolerance to (ecological) characteristics. The usefulness of this powerful method should become even clearer when substantially higher numbers of individuals are analysed (as facilitated by using DNA barcodes) and when missing data are filled in