Soil contains a wealth of diversity – bacteria, fungi, nematodes, arthropods and earthworms are just some of the many organisms found belowground. These organisms play an important role in shaping the soil environment and they strongly influence plant fitness, diversity and community composition. Their impact even cascades up to affect aboveground species interactions. Ultimately, belowground organisms are a vital part of ecosystem functioning. Nevertheless, most of the diversity and ecology of belowground organisms are to this day unknown, and increasing our insights into the role and ecology of soil organisms is of importance for natural and agricultural systems.

The main goal of this thesis was to investigate spatial patterns of plant-associated soil communities (I, II), to identify the drivers of such spatial patterns (I, II, III), and to study some of the consequences of belowground spatial patterns for aboveground species interactions (IV). To answer these questions, I used both observational studies and multifactorial experiments in combination with microscopy and metabarcoding. I focused on the plant Plantago lanceolata (ribwort plantain) and its root-associated soil microbes, with a strong emphasis on arbuscular mycorrhizal fungi, an important group of root symbionts.

I found that in natural environments arbuscular mycorrhizal fungal communities frequently show high small-scale variation (I). In the following work I showed that the pattern of high small-scale heterogeneity may be due to dispersal limitation (II), abiotic conditions such as pH, soil nutrients and climate (I, III), and biotic conditions, such as interspecific community composition and genetic variation (I, II). The high variation at small spatial scales (I) in combination with genetic variation of plants and insects (IV) may help maintain high local heterogeneity in aboveground plant-associated communities, thereby influencing aboveground diversity and dynamics.

The insight gained here has increased our general knowledge on the distribution of soil microbes and the interactions taking place above and belowground. It has furthermore laid a foundation for future work on the world of soil microbes and their implications aboveground.