Effects of six grassland plant species on soil nematodes: A glasshouse experiment
Introduction
Plant species identity and the composition of the plant community are important determinants of decomposer function and community composition in soil (Wardle, 2005). For microorganisms, this has been shown both in terms of microbial biomass and activity in microcosms with soil from grasslands (Bardgett et al., 1999; Innes et al., 2004) as well as microbial community composition in microcosms (Grayston et al., 1998; Marschner et al., 2001), field experiments (e.g., Smalla et al., 2001; Kowalchuk et al., 2002) and in a naturally mixed spruce-birch stand (Saetre and Bååth, 2000).
Soil fauna are also affected by individual plant species. In both field and pot experiments, nematodes responded to the identity of different grassland species, with plant-feeding and microbial-feeding nematodes (bacterial and fungal feeders) being the groups most clearly influenced by plant species identity (Wardle et al., 2003; De Deyn et al., 2004; Viketoft et al., 2005). Collembolans have been shown to respond to grass and legume species in pot and field experiments with an assembly of grassland species (Salamon et al., 2004; Milcu et al., 2006), and both mites and collembolans responded to plant species in high Arctic heath vegetation (Coulson et al., 2003). In addition, mites have also been shown to be affected by different plant species used in agroforestry (Badejo and Tian, 1999).
Plants affect the belowground community through differences in the amount and quality of resources allocated to the soil, in the extent to which they deplete nutrients and water from the soil, in the chemical composition of the litter produced and through the formation and modification of habitats (Wardle, 2002). For example, many legumes differ from other forbs in their ability to fix atmospheric nitrogen and therefore increase the amount of nitrogen in the soil, principally in the form of nitrate (Palmborg et al., 2005). They also produce more high-quality litter that is more easily decomposed (Scherer-Lorenzen et al., 2003). Previous studies on the effect of plant species on nematodes have shown that, in general, plant feeders seem to be most abundant under grasses and have a lower presence under some forbs, while microbial feeders show varying responses to the investigated plant species in different studies (Wardle et al., 2003; De Deyn et al., 2004; Viketoft et al., 2005). In addition, different feeding types of plant feeders, e.g., ecto- and endoparasites, have been shown to respond differently to grasses and forbs (De Deyn et al., 2004).
The aim of the present study was to examine, in a glasshouse pot experiment, how six plant species, belonging to three plant functional groups (grasses, legumes and forbs), affect total nematode abundance, nematode community composition and the response of individual nematode genera. A defaunated soil from an experimental grassland that was reinoculated with both microflora and nematodes was used. Thus, the effect of plant species during the establishment phase of nematode communities was evaluated under controlled conditions. In addition, a treatment containing no plants was included, top-down effects by micro- and mesofauna other than nematodes were removed and soil characteristics (e.g., pH, inorganic nitrogen), missing in other studies (Wardle et al., 2003; De Deyn et al., 2004; Viketoft et al., 2005), were determined to relate to the nematode data.
Specifically, the following hypotheses are addressed:
- 1.
Low numbers of nematodes should be found in the treatment with no plants, due to the lack of plant-derived resources.
- 2.
There is a positive correlation between total nematode abundance and plant productivity (Yeates, 1987).
- 3.
Plant-feeding nematodes should show the strongest response to the identity of plant species because they feed directly on them, although the speed of the response between bacterial- and plant feeders likely differs because of different generation times. Based on previous studies, plant feeders are also expected to be most abundant under grasses.
- 4.
Legumes, due to their N-fixing ability, will affect the nitrogen levels in the soil and hence the bacterial community. This should affect the nematode community composition by promotion of bacterial-feeding nematodes, especially Rhabditis and Panagrolaimus.
- 5.
Different plant species within a functional group will have quite different influences on the nematodes (Viketoft et al., 2005). The division into plant functional groups was based on growth form and N-fixing ability, but the plant species may contain different secondary substances.
The nematode fauna under monocultures of the plant species used in this study have previously been investigated after seven growing seasons in an experimental grassland in northern Sweden (Viketoft et al., 2005). Although the present study is a 16-week experiment, where plants were getting established and a limited number of nematodes were inoculated, it had higher replication and a treatment containing no plants compared to the field experiment. A comparison between these studies may illustrate interesting aspects of the interactions between plants and soil communities.
Section snippets
Site and soil
In November 2004, soil was collected to 10 cm depth from the experimental field of the BIODEPTH project of the Swedish University of Agricultural Sciences, Umeå, northern Sweden (63°45′N, 20°17′E). The soil is classified as silt loam (4.1% clay, 57.9% silt, 38.0% fine sand) and was taken from an area between the experimental plots, which is dominated by Phleum pratense and Trifolium repens. The soil was stored at 4 °C until further processed in February 2005. The vegetation was then removed and
Plant growth and soil chemistry
Shoot and root biomass differed significantly between the plant treatments (Table 1). In general, the legumes had the highest shoot biomass and forbs the lowest. No such pattern among plant functional groups was found for the root biomass. Instead, R. acetosa clearly had the greatest root biomass. The presence of plants affected the soil during the experimental period, which is demonstrated by the pots without plants having the highest nitrate levels and the lowest organic matter content (Table
Discussion
As hypothesised, the no-plant treatment had the lowest total number of nematodes. However, a correlation between total nematode abundance and plant biomass was not found, but the abundance of bacterial feeders correlated with both shoot and root biomass. Plant-feeding nematodes showed the strongest response to the different plant species but in contrast to the hypothesis they did not have their greatest abundances under grasses. In addition, legumes did not specifically promote
Acknowledgements
The author thanks Cecilia Palmborg for collecting the soil, Karin Önneby for help with the set-up of the experiment and Birgitta Vegerfors-Persson for statistical advice. Author also thanks Jan Bengtsson, Björn Sohlenius and Cecilia Palmborg for valuable comments on previous versions of the manuscript and the anonymous reviewers for constructive comments on the manuscript. This study was funded by the Oscar and Lili Lamm Foundation and the Swedish Research Council (grant to Jan Bengtsson).
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