Local variation in belowground multitrophic interactions

A growing number of studies point at the involvement of root herbivores in influencing plant performance, community composition and succession. However, little is known about the factors that control root herbivore abundance and the role of local variation in the effectiveness of these factors. Here, we performed a full factorial experiment with plants, root-feeding nematodes and rhizosphere microbial communities from two dune sites, to test the hypothesis that the outcome of belowground multitrophic interactions depends on local differences between the interacting organisms. The organisms included the marram grass Ammophila arenaria, the cyst nematode Heterodera arenaria, microbial plant pathogens and natural enemies of the nematodes from two coastal foredune systems, one in The Netherlands and one in Wales. The two plant populations differed at the molecular and phenotypic level, and the microbial communities from the two dune sites differed in the composition of the dominant soil fungi but not of the dominant bacteria. Plants were negatively affected by the rhizosphere microorganisms from one of the sites. Nevertheless, nematode performance was not affected by the origin of both the host plants and the microbial communities. The reproductive output of the cyst nematode depended on the presence of microorganisms, as well as on inter-population variability in the response of the nematode to these natural enemies. In the absence of microorganisms, the two nematode populations differed in the number and size of the produced cysts, although maternal effects cannot be excluded. Inter-population differences in the host plant were a secondary factor in the nematode–microorganisms interactions, and did not influence bottom–up control of the cyst nematodes. Our results did not reveal strong signals of coevolution in belowground multitrophic interactions of plants, cyst nematodes and soil microbial communities. We conclude that the interactions between the studied organisms do not necessarily depend on their local vs. non-local origin. Nevertheless, we were able to show that local variation in soil organism community composition can be an important factor in determining the outcome of interactions in belowground multitrophic systems.