Two new studies shed light on how the composition of biota in soil drives plant diversity. The results highlight how the unseen bacterial and fungal organisms below ground influence the development of plants above, a relationship that remains poorly understood. In the first study, Jonathan Bennett et al. explored how the presence of arbuscular mycorrhizas (AMs) and ectomycorrhizas (EMs) in soil affect the biodiversity of trees in North America. Mature trees are known to influence the composition of these fungi within the soil, which in turn can influence whether saplings of the same species are also able to take root. For example, the accumulation of antagonists (such as soil-borne herbivores and pathogens) near adult plants can increase local diversity by reducing recruitment of similar species. Or, the accumulation of mutualists can increase the dominance of one species, reducing diversity. Here, Bennett and colleagues sampled 55 North American temperate tree species (30 EM and 25 AM) from 550 geographically distinct populations. Their analysis suggests that EM seedlings benefit from a greater abundance of EM fungi near mature trees of the same species, whereas AM seedlings are subjected to more antagonists when near trees of the same species. AM seedlings were found to have more lesions; thus, the authors propose that AM offers less protection than colonization by EM fungi. Through a series of experiments where seedlings were pre-exposed to the two fungi, the researchers found that EM, but not AM, fungi reduce root damage and improve survival.
In a separate study, François P. Teste and colleagues collected soil surrounding plants that rely on five different nutrient-deriving techniques: AM, EM, ericoid mycorrhizal (ErM), nitrogen-fixing (NF) bacteria, and nonmycorrhizal cluster-rooted (NMCR). They grew 16 species of Australian plants in different combinations of these biota-filled soils. Their results reveal that NF and NMCR plants are less likely to survive in soil that contains all biota. These types of plants, when they did survive, experienced reduced growth. In contrast, survival of ectomycorrhizal plants was enhanced in soil with all biota present. In sterile soil void of biota, arbuscular, ectomycorrhizal and NF plants underwent reduced growth, as they had limited root colonization by mycorrhizal fungi. Lastly, the researchers ran simulations where soil biota were absent using the survival and growth data from sterilized soil, finding that plant species and functional diversity declined rapidly to unrealistically low levels when no biota are present. These two studies are highlighted in a Perspective by Wim H. van der Putten.