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
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.