Root-fungal interactions upon resource foraging in heterogeneous soils

Plants respond to soil resource heterogeneity by foraging with their roots. The root foraging precision of plant species varies; and some species (especially those with low precision) may rely on symbiosis with mycorrhizal fungi to access nutrient-rich patches, thereby improving nutrient acquisition. However, the way herbaceous plants and arbuscular mycorrhizal fungi (AMF) interact during nutrient foraging is poorly understood.

We tested nine mycorrhizal herbaceous plant species in a greenhouse experiment by exposing them to soil with heterogeneous or homogeneous nutrient distribution and with or without AMF inoculation. Root-foraging precision, shoot and root biomass production, and root-associated AMF gene copy number were quantified using plant biomass measurements and qPCR analyses of AMF.

Root foraging precision (in terms of roots placement) differed between species. Although foraging was evident in the heterogeneous nutrient treatment, it was largely unaffected by mycorrhizal inoculation. The exception was Melilotus officinalis which showed reduced root foraging precision with increasing root-associated AMF gene copy number. Plant biomass responses to AMF were species-specific, with overall root and shoot biomass declining slightly with increasing root-associated AMF gene copy number.

These results suggest that AMF inoculation generally does not alter root foraging precision across species, supporting the view that foraging precision is a species-specific trait independent of the collaboration gradient in the root economic spectrum. The balance between root foraging and mycorrhizal symbiosis may depend on other factors such as nutrient stoichiometry and forms. Future studies should thus examine root foraging, extraradical hyphal development and nutrient transfer under varying levels of nutrient limitation and spatial heterogeneity to better understand plants' nutrient acquisition strategies.