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International audience; 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. (c) 2010 Elsevier Ltd. All rights reserved.

Induced resistance (IR) offers a lasting defense against a broad spectrum of diseases in various plants, but practical applicability is impaired by unpredictable effectiveness. Here we present the results of two greenhouse experiments on the modification of the effectiveness of BION® (acibenzolar-S-methyl (BTH)) induced resistance against powdery mildew in barley by different levels of root infection with mycorrhizal fungi (Glomus etunicatum) and parasitic nematodes (Heterodera avenae), respectively. Root infection by mycorrhizal fungi and nematodes both had a negative effect on plant growth. Adverse effects of G. etunicatum suggest that parasitic effects of mycorrhizal fungi outweighed positive effects under the optimal growth conditions provided by the experiment. Bion® either did not affect or decreased the number of lesions caused by powdery mildew at low and medium levels of infection by G. etunicatum or H. avenae, but increased it at high levels of root infection. This clearly proves the important role of root-associated biota in modulating the effectiveness of IR. However, plant accumulation of the resistance related protein BCI-4 was not impaired by G. etunicatum or H. avenae. Soil biota thus appear to impact later steps of the defense pathway, rather than preventing the onset of resistance.