Plant-soil feedbacks contribute to an intransitive competitive network that promotes both genetic and species diversity.
Plant communities are generally thought to follow strict competitive hierarchies, in which species can be linearly ordered according to their ability to compete for a few limiting resources. Such strict hierarchies should lead to reduced diversity, since the single best competitor will eventually exclude the other species. However, more complex dynamics may emerge if different plant species or genotypes compete in different ways, such as through the release of toxic allelochemicals or the alteration of soil microbial communities. Brassica nigra genotypes and their competitor species show a 'rock-paper-scissors' like dynamic, in which no one type is competitively superior to all others. This intransitive dynamic can maintain both species diversity and genetic variation in an allelochemical trait of B. nigra. Here we show that feedbacks with soil biota, probably involving arbuscular mycorrhizal fungi, facilitate these dynamics. Brassica nigra genotypes that invest heavily in the allelochemical sinigrin reduce mycorrhizal abundance of surrounding soils, which reduces the growth of heterospecific competitors. Since B. nigra is non-mycorrhizal, investment in sinigrin does not improve a genotype's ability to compete with other conspecifics. Synthesis. These results highlight the importance of complex soil communities for the maintenance of both genetic and species diversity in plant communities.