Plant–soil feedbacks of plants that are exposed to herbivory have been shown to differ from those of plants that are not exposed to herbivores. Likely, this process is mediated by jasmonic acid (JA) and salicylic acid (SA) defense pathways, which are induced by aboveground herbivory. Furthermore, exogenous application of these phytohormones to plants alters belowground communities, but whether this changes plant–soil feedbacks in natural systems is unknown. We applied exogenous sprays of JA and SA individually and in combination to field plots in a restored grassland. Control plots were sprayed with demineralized water. After three repeated application rounds, we transplanted seedlings of the plant–soil feedback model plant Jacobaea vulgaris as phytometer plants to test the effects of potential phytohormone-mediated changes in the soil, on plant performance during the response phase. We further measured how exogenous application of phytohormones altered plant-related ecosystem characteristics (plot-level); soil chemistry, plot productivity, insect communities and predation. Biomass of the phytometer plants only co-varied with plot productivity, but was not influenced by phytohormone applications. However, we did observe compound-specific effects of SA application on insect communities, most notably on parasitoid attraction, and of JA application on soil nitrogen levels. Although we did not find effects on plant–soil feedbacks, the effects of exogenous application of phytohormones did alter other ecosystem-level processes related to soil nutrient cycling, which may lead to legacy effects in the longer term. Furthermore, exogenous application of phytohormones led to altered attraction of specific insect groups

Plant biomass was collected: by uprooting focal plants (Jacobaea vulgaris) and separating above and belowground biomass by clipping all plot (60x60cm) biomass aboveground at ground level, from this, the productivity was calculated per square meter. Insects were collected; by 10x10 cm blue sticky traps that were placed in the middle of each plot and left for 24 hours. Insects were identified to order and feeding guild. Soil chemistry was determined; following details in the methods of Heinen et al. 2020 Arhtropod-Plant Interactions Predation was assayed; using clay model caterpillars (4 per plot) that were visually checked under stereomicroscope for bitemarks of birds, mammals, arthropods or slugs.