Effects of above and belowground biodiversity on the metabolome of common grassland species in different environmental contexts The loss of biodiversity is negatively affecting ecosystem functions and biotic interactions. However it is those biotic interactions that are essential for ecosystem fitness productivity and resilience. In terrestrial ecosystems the multitrophic above belowground interactions among plants their soil biota and the aboveground herbivore community drive many important ecosystem processes. While we know of the close connections between plants and their soil biota community or plants and their herbivore community we still lack the mechanistic understanding underlying these biotic interactions. It is here that the newly emerging research area of ecological metabolomics (eco metabolomics) may provide new insights by uncovering the molecular processes of plant soil herbivore interactions. My dissertation aimed to provide insights into the molecular mechanisms underlying plant soil herbivore interaction by combining the application of eco metabolomics with biodiversity experiments. I combined controlled pot experiments with a field experiment in a semi natural grassland to test whether different levels of plant diversity and soil biota diversity affect plant metabolomes. Furthermore I tested if these changes in plant metabolomes and the diversity of secondary metabolites can be linked to aboveground herbivory to explain variation therein. My dissertation provides experimental evidence that plant diversity can induce shifts in soil biota diversity that can affect the plant’s metabolome as well as the concentration and diversity of secondary metabolites. In addition my results suggest that the plant’s metabolome is a novel and important functional trait that can link plant diversity and herbivory and explain variation thereof. By including metabolomic analyses we gained a better understanding for the importance of plant soil interactions in shaping ecosystem processes such as aboveground plant herbivore interactions via their effect on the plant’s metabolome.