The productivity and diversity of plant communities are affected by soil organisms such as arbuscular mycorrhizal fungi (AMF),
root herbivores and decomposers. However, it is unknown how interactions between such functionally dissimilar soil organisms
affect plant communities and whether the combined effects are additive or interactive. In a greenhouse experiment we investigated
the individual and combined effects of AMF (five Glomus species), root herbivores (wireworms and nematodes) and decomposers
(collembolans and enchytraeids) on the productivity and nutrient content of a model grassland plant community as well as on
soil microbial biomass and community structure. The effects of the soil organisms on productivity (total plant biomass), total
root biomass, grass and forb biomass, and nutrient uptake of the plant community were additive. AMF decreased, decomposers
increased and root herbivores had no effect on productivity, but in combination the additive effects canceled each other out.
AMF reduced total root biomass by 18%, but decomposers increased it by 25%, leading to no net effect on total root biomass
in the combined treatments. Total shoot biomass was reduced by 14% by root herbivores and affected by an interaction between
AMF and decomposers where decomposers had a positive impact on shoot growth only in presence of AMF. AMF increased the shoot
biomass of forbs, but reduced the shoot biomass of grasses, while root herbivores only reduced the shoot biomass of grasses.
Interactive effects of the soil organisms were detected on the shoot biomasses of Lotus corniculatus, Plantago lanceolata,
and Agrostis capillaris. The C/N ratio of the plant community was affected by AMF.
In soil, AMF promoted abundances of
bacterial, actinomycete, saprophytic and AMF fatty acid markers. Decomposers alone decreased bacterial and actinomycete fatty
acids abundances but when decomposers were interacting with herbivores those abundances were increased. Our results suggests
that at higher resolutions, i.e. on the levels of individual plant species and the microbial community, interactive effects
are common but do not affect the overall productivity and nutrient uptake of a grassland plant community, which is mainly
affected by additive effects of functionally dissimilar soil organisms.