Terrestrial nematodes in a changing environment

Increasing awareness of the nature and extent of soil pollution on soil biota and their role in soil processes has resulted in exploring the possibilities of biological assessment systems to indicate the ecological condition of soils and to predict the ecological efficacy of e.g. policy measures. The research presented in this thesis is part of a project in which the possibilities of nematodes as bioindicator organisms are studied. The objectives of the study were to investigate i. the use of the nematode fauna to assess soil quality, and ii. the prospect of the nematode fauna to contributing to an ecological soil classification which can serve as a reference to assess soil quality. An ecological soil classification is based on a description of soils consisting of a set of both biotic and abiotic parameters.Besides the effects of certain pesticides on the occurrence of nematodes, many studies within the last decades showed changes in the composition of the nematode fauna due to e.g. heavy metals and other pollutants, fertilization and acidification. To assess soil quality by means of analyzing the composition of the nematode fauna, an effective and reliable instrument is needed. In Chapter 2 an assessment method based on life strategies of nematodes is described. Nematode taxa. can be classified from colonisers (r-strategists in the loose sense) to persisters (K-strategists sensu lato ) on a c-p scale. This c-p scale was divided into three classes, and of any soil sample the proportion of the nematode fauna belonging to each of these three classes was calculated. C-P triangles were used to visualize the distribution of the three c-p classes within a sample. It was shown that c-p triangles could be used to describe patterns in the composition of the nematode fauna which were found to be related to certain disturbances such as acute toxic compounds (arrow 2 Fig. 2.4) or those resulting in an increased availability of easily decomposable organic compounds (arrow 1 Fig. 2.4).In Chapter 3, c-p triangles were applied to study changes in the composition of the nematode fauna of a Scots pine forest, resulting from the application of lime, additional nutrients and organic manure to restore nutrient availability for tree growth. Soil samples were taken three and four years after the first applications, and in both years the occurrence of nematodes was significantly effected by liming. Analyses of patterns within the c-p triangles based on our own data and on a selection of literature data originating from locations with different nematode faunae showed that following liming, colonizers increased proportionally. This increase became manifest months later and could be demonstrated several years later. Application of urea also resulted in an increased abundance of colonizers. However, the effects of urea were stronger, appeared within a few weeks of application, but seemed to last for only a few months.In experimental studies like those described in Chapter 3, control treatments usually are available. In general, however, the assessment of the pollution or disturbance of soils lacks such controls. A reference system (e.g. ecological soil classification) or an analysis of changes in the composition of the nematode fauna using a time sequence, can then be used to assess the nematode fauna of that soil.Development of a reference system depends upon, among other things, knowledge of the ecological relationships of nematodes to their biotic and abiotic environment. Relationships between soil characteristics, vegetation and composition of the nematode fauna are described in Chapter 4, where the nematode fauna of >200 soil samples taken from a variety of habitats differing in vegetation (forest, shrubs, heathland, grassland) and soil type (clay, loam, sand) was studied. Using multivariate analysis techniques (clustering, correspondence analyses), the nematode fauna of these sites was classified in "Sample Groups". Seven Sample Groups could be distinguished, and these could also be described by soil characteristics in combination with vegetation. Gradual changes in the composition of the nematode fauna were observed in the sequence sandy, loamy sandy, sandy loam, loam soils. However, the nematode fauna of clay soils differed significantly from the former. In soils with comparable physical and chemical characteristics, subdivision was found to he related to vegetation, as was shown for the sandy soils. In sandy soils both the forests and the forest gaps, shrubs and grasslands had their characteristic nematode fauna. Methodological aspects as date of sampling and sampling strategy did not affect the Sample Group classification. It was concluded that the nematode fauna can contribute to a reference system such as an ecological soil classification.Assessment of changes in the composition of the nematode fauna in monitoring studies requires knowledge of the development of nematode faunae in natural habitats. Long-term changes in the composition of the nematode fauna were studied in a natural primary vegetation succession of blown-out areas in a drift sand landscape (Chapters 5 and 6). The successional stages studied were bare drift sand, Spergulo-Corynephoretum, Scots pine ( Pinus sylvestris L.) forests of respectively 3-5, 25-30, 45-50, 80-90 and 105 years old and an early variant of Betulo- Quercetum. Nematode samples were taken from the 0-10 cm mineral soil, and in the forested stages also from the litter, fermentation and humus horizons.The occurrence of nematodes depended on the stage of succession, the vegetation type (drift sand, Spergulo-Corynephoretum, forest) and the soil horizon, and could be related to microclimatological characteristics, soil texture and food availability. Relatively large differences in the composition of the nematode fauna were observed when going from drift sand to Spergulo-Corynephoretum, and subsequently to Scots pine forest. Changes in the composition of the nematode fauna during forest development appeared more gradual.Analyses of the nematode fauna of the forested sites showed a higher similarity between the nematode faunae of comparable soil horizons than between different horizons within the same profile. Within each soil horizon, c-p triangles showed patterns in nematode fauna development which could be related to vegetation succession and which differed per horizon. In the 0-10 cm mineral soil and in the fermentation and humnus horizons of the forested soils nematode development followed patterns as indicated by arrow 4 in Fig. 2.4: an initial dominance of taxa belonging to c-p group 2, followed by a relative increase of persisters (c-p group 3-5). However, in the litter horizons such development of persisters, could not be observed. In all successional stages of the litter horizon, the nematode fauna was composed mainly of colonizers (c-p groups 1 and 2), and here the most extreme colonizers (c-p group 1), could be found in relative high numbers.Finally, the nematode fauna in the natural vegetation succession of the blown-out areas in the drift sand landscape was compared with the Sample Group classification of Chapter 4 (Fig. 5.7). Projection of the nematode samples taken from the vegetation succession onto the correspondence analysis graph of the Sample Group data, showed that the position of the successional samples coincided with the Sample Groups composed of samples with related habitat types. These similarities between nematode fauna structure and habitat type as found in both investigations, support a classification of soils based on the composition of the nematode fauna as presented in Chapter 4. Moreover, with reference to an ecological soil classification, this comparison showed that developmental changes of the nematode fauna as a result of natural environmental changes in habitat structure can cross boundaries between "classes" or "types"