Variability of arbuscular mycorrhizal fungal communities within the root systems of individual plants is high and influenced by host species and root phosphorus

The spatial distribution of arbuscular mycorrhizal (AM) fungal communities has been studied at many scales, from global to local, but few studies have explored variation within the root system of individual plants. Spatial variation within root systems might be expected partly due to differences in the physiology, morphology and anatomy of different roots, as well as due to spatial variation in the composition of inoculum in soil. Here, we assessed AM fungal community variability at various local scales: within individual plant root systems and among plants separated by distances of a few centimetres to up to several metres. To do so, we sampled roots from four plant species (two grasses and two forbs) growing in a grassland experiencing experimentally altered rainfall patterns in replicated field plots. We found that AM fungal communities were highly dissimilar both within the same individual and among different plants. Communities sampled within the same root system were as dissimilar as those sampled from adjacent neighbours, but less dissimilar from those sampled at larger distances. The degree of within root system variability was as high for each watering treatment but varied among plant species in a way that was dependent on root phosphorus content. Overall, our results show that community dissimilarity levels are high even within the root systems of individual plants, possibly due to localised community assembly processes (e.g., priority effects and interactions among AM fungal species), but that the degree of homogenisation within a root system is partly dependent on host plant attributes.