Conservation of biodiversity depends on understanding the mechanisms that determine species richness and their relative abundances. The basic scientific question on which this IRG proposal is based is why are some plant species more abundant than others? Different theories have been proposed to explain the mechanisms of plant coexistence. However, not only do they not provide a satisfactory answer, but also, they largely ignore an increasingly recognized important component; the roles of the soil biota. A large part of a plant is its root system which grows in soil, a matrix that, due to its characteristics, serves as habitat for a huge diversity of organisms, many of which interacting with plants. This proposal focuses on the outcomes of these interactions, particularly on identifying the role of equilibriums between beneficial and damaging interactions and to which extent the role of the former ones is to keep the latter ones at bay. It is predicted that the outcomes of these interactions vary depending on plant traits, ultimately determining a species relative abundance. The IRG will focus on a target natural grassland community and on the arbuscular mycorrhizal (AM) symbiosis, which is widespread among plants. A combination of molecular and classical techniques will be used to investigate whether abundant plants receive greater pathogen protection from the AM symbiosis than rare plants. The IRG will be conducted in collaboration with experts from the Univ. of Potsdam in the area of theoretical plant ecology, which is complementary to the applicant’s expertise. It is expected that the results will increase our understanding of the role of soil-plant-microbe interactions in ecology and their potential to be managed to prevent the loss of plant species. Furthermore, by promoting collaboration this IRG will enable transferring knowledge acquired by the applicant during the past seven years in Canada, thereby significantly promoting his long-term reintegration.