Soil nematode fauna of a subarctic heath: potential nematicidal action of plant leaf extracts

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Dwarf shrub heaths are common vegetation types of the Arctic and Subarctic. Some of the dominant plants in these communities release toxic substances which can affect neighbouring plants. We investigated the effects of leaf extracts from two dwarf shrubs, Cassiope tetragona and Empetrum hermaphroditum, and from mountain birch, Betula pubescens spp. tortuosa, on nematode community structure and reproduction in a pot and an agar culture experiment. In pots two graminoids, Carex bigelowii and Festuca viuipara, were grown in previously sterilized or non-sterilized soil. Soil was collected from a subarctic heath, and plants from a fellfield site, near Abisko, northern Swedish Lapland. The naturally occurring nematode community at the heath site is also described. A total of 34 nematode taxa were observed at the field site and 32 taxa in the pot experiment. Most species, especially frequent taxa, occurred in both soils. Potted soil was strongly dominated by bacterial feeders and showed a shift towards a more colonizer controlled population. These changes were presumably due to the establishment of residual species after freezing and/or sterilization of the soil prior to incubation in the laboratory. Direct exposure of the nematode Aphelenchoides saprophilus to the leaf extracts in agar cultures resulted in a lower reproduction rate. Leaf extract treatment in potted soil altered nematode species composition and dominance structure, and generally reduced species number and maturity index. The diversity of genera was decreased by all extract treatments in pots with Carex. Nematode population density was not affected, except after application of Betula extracts, where significantly higher densities occurred. Nematode data were related to respiration and active fungal biomass in the soil, indicating that rather than allelochemicals with nematicidal properties, carbohydrates in the leaf extracts promoting a beneficial microflora may have affected the nematode community.