Effects of plant species and plant diversity on soil nematodes - a field experiment on grassland run for seven years

Plant species identity and diversity may greatly influence the composition of the nematode fauna. In this study the development of the nematode fauna was followed in a field experiment on arable soil with monocultures and mixtures of several plant species. Experimental plots were sown with one, four or 12 species of grass, legumes and forbs and were sampled four times in 7 years. Nematode diversity was little influenced by plant diversity. Due to a pronounced increase of Paratylenchus project us populations, the Shannon diversity index decreased in several treatments towards the end of the experiment. Differences in nematode faunal composition among treatments increased with time. In spite of the rather long duration of the experiment, the faunal composition did not stabilise but changed continuously. The obligate plant feeders Tylenchorhynchus dubius, T maximus and Pratylenchus spp. occurred in higher numbers in monocultures than in mixtures of several plant species. Among the microbivores, the abundance of some bacterial- and fungal-feeding nematode taxa seemed to be directly influenced by the plant species identity. In the plots with Trifolium spp. there was an increase of rapidly growing bacterial feeders belonging to Rhabditidae and Panagrolaimidae already after the first growing season. The abundances of some bacterial-feeding nematodes were correlated to total plant production. The numbers of the omnivorous Mesodorylaimus sp. appeared to be influenced by the degree of plant coverage. The abundance of several nematode taxa appeared to be little influenced by the composition of the vegetation. The plant feeder P. projectus and the bacterial feeder Prismatolaimus sp. reacted rather slowly and a distinct increase in numbers was only seen after 7 years, when P projectus strongly dominated the fauna in several treatments. Among the bacterial feeders, some species with moderate growth rate belonging to Cephalobidae decreased with time in several treatments.