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Plant diversity is known to influence the abundance and diversity of belowground biota; however, patterns are not well predictable and there is still much unknown about the driving mechanisms.
We analyzed changes in soil nematode community composition as affected by long-term manipulations of
plant species and functional group diversity in a field experiment with plant species diversity controlled
by sowing a range of 1–60 species mixtures and controlling non-sown species by hand weeding. Nematode
communities contain a variety of species feeding on bacteria, fungi, plants, invertebrates, while some are
omnivorous. We analyzed responses of nematode abundance and diversity to plant species and functional
diversity, and used structural equation modeling (SEM) to explore the possible mechanisms underlying the
observed patterns. The abundance of individuals of all nematode feeding types, except for predatory
nematodes, increased with both plant species and plant functional group diversity. The abundance of
microbial-feeding nematodes was related positively to aboveground plant community biomass, whereas
abundance of plant-feeding nematodes was related positively to shoot C:N ratio. The abundance of predatory
nematodes, in turn, was positively related to numbers of plant-feeding nematodes, but not to the
abundance of microbial feeders. Interestingly, the numbers of plant-feeding nematodes per unit root mass
were lowest in the high-diversity plant communities, pointing at reduced exposure to belowground herbivores
when plants grow in species-diverse communities. Taxon richness of plant-feeding and microbialfeeding
nematodes increased with plant species and plant functional group diversity. Increasing plant
functional group diversity also enhanced taxon richness of predatory nematodes. The SEM suggests that
bottom-up control effects of plant species and plant functional group diversity on abundance of nematodes
in the various feeding types predominantly involve mechanistic linkages related to plant quality instead of
plant quantity; especially, C:N ratios of the shoot tissues, and/or effects of plants on the soil habitat, rather
than shoot quantity explained nematode abundance. Although aboveground plant properties may only
partly serve as a proxy for belowground resource quality and quantity, our results encourage further
studies on nematode responses to variations in plant species and plant functional diversity in relation to both quantity and quality of the belowground resources.
