Competitive dynamics of arbuscular mycorrhizal fungi as depending on fungal traits and host plant species

Arbuscular mycorrhizal fungi (AMF) are ubiquitous root-associated symbionts, but competitive interactions among coexisting taxa remain poorly understood. The variation in colonization and resource-acquisition strategies drive competition, shaping the relative abundances of AMF within their communities and mycorrhiza functioning. However, the factors that determine the competitive outcomes have received limited experimental attention. We hypothesized that AMF abundances, competitive responses, and contributions to plant growth would vary according to fungal traits and host plant species. Three AMF isolates of contrasting growth and competitive abilities, each representing one AMF species, were mono- and co-inoculated under six host plant species. Fungal abundance, root colonization (RC), and mycorrhizal growth response (MGR) were measured at early and late stages of the symbiosis. Fungal competitive responses depended on the isolate’s root colonization strategy in monoculture and mycorrhizal stage. The fast-colonizing isolate dominated initially, while slower-growing isolates displayed different temporal patterns, either declining or maintaining their abundance over time. Our findings suggest competitive outcomes among AMF that were asymmetric at early stages but became more symmetric later. The host plant species importantly modulated the dominance of the fast root-colonizer. MGR was positively associated with total AMF abundance and RC, yet the most abundant isolate did not necessarily provide the greatest growth benefits to the host plant. Our results suggest that maintaining a diversity of AMF functional types, rather than introducing a single highly competitive isolate, is favourable to the establishment of stable and efficient plant-AMF associations.