Mycorrhizal growth and resource exchange define plant’s coexistence

Symbiotic interactions between roots and mycorrhiza drive plant coexistence, yet the roles of the common mycorrhizal network (CMNs) between plant species remain poorly understood.

We conducted a compartmented microcosm experiment to assess AMF effects on the coexistence of mixed legumes (Medicago sativa or Trifolium repens) with grasses (Dactylis glomerata). Plant species were selected based on distinct functional characteristics such as symbiotic N₂-fixation ability, plant height, rooting depth, root diameter, and root surface area. The δ¹³C signature of AMF-specific fatty acids (C16:1ω5) in the hyphal compartment were measured to determine the carbon contribution of symbiotic plants in CMNs. Dual-labeled organic substrates (¹³C:¹⁵N) were used to assess the organic nitrogen uptake by host plants through CMNs.

Plant coexistence depended on the mycorrhizal growth response (MGR) of host plants and the resource (C and N) exchange through CMNs, as plants benefit specifically from their fungal partners. MGR was closely correlated with plant functional traits. Legumes had a greater MGR than grasses, primarily due to their thicker roots and smaller root surface area. In M. sativa + D. glomerata bi-mixture, M. sativa, with a greater MGR in biomass and net photosynthetic rate, consistently invested more carbon for the fungal partners. Simultaneously, both M. sativa and D. glomerata obtained nutritional benefits from the hyphal network. Carbon allocation and nutrient acquisition were more balanced in mixtures T. repens + D. glomerata.

Differential MGR of host plant growth and the exchange of resources through CMNs are crucial for the coexistence of plant species in mixtures.