Anika Lehmann, Matthias C. Rillig

Mycorrhizal fungi can interlink and connect plants in a common mycorrhizal network (CMN). Studying CMNs is challenging due to pathways of material transfer but also plant and mycorrhizal effects that have to be tested and controlled in order to be able to evaluate the presence and magnitude of a specific CMN effect. These controls let to a clear but strict definition of CMN which requires experiments to fulfill specific criteria: at least two plants are connected by the CMN, all plants are mycorrhized, the roots of the connected plants are separated, there is a CMN treatment tested, and the hyphal continuity is tested.
Here, we evaluate the evidence base of the CMN research specifically for arbuscular mycorrhiza via a systematic mapping approach. We found that not all studies were testing true CMNs but rather common fungal networks (CFN), including filamentous fungi other than the targeted mycorrhizal fungi. The number of articles conducting experiments on CMNs drops strongly with increasingly stringent fulfillment of the CMN definition. Additionally, there is a focus on lab studies and specific fungal strains; however, researchers have used diverse plant species setups. Also plant, fungal and resource transfer responses are preferentially measured, while microbial community metrics and ecosystem functions and processes are neglected.
We see a need to strengthen the CMN evidence base and thus we call for a renewed research effort on CMN, focusing on a whole range of levels of mechanistic resolution (from CFN to CMN with and without hyphal continuity). Additionally, neglected experimental situations (e.g. field studies in general) and microbial community or ecosystem-level responses should be included in future research.