Agnostic fungi: plant traits and tissue stoichiometry explain nutrient transfer in common arbuscular mycorrhizal networks of temperate grasslands

Plants and mycorrhizal fungi form close mutualistic relationships that affect the structure and function of ecosystems. Common mycorrhizal networks (many plants associated with the same fungus) can facilitate preferential transfer of carbon and limiting nutrients. The mechanisms behind these networks remain poorly understood. Do common mycorrhizal networks favor fungal growth at the expense of plant resource demands (a fungi-centric view), or are they passive channels through which plants regulate resource fluxes (a plant-centric view)? In experimental restored prairie and introduced pasture systems in the Pacific Northwest, USA, we used stable isotope tracers (13C and 15N), paired with analyses of plant traits and fungal community DNA, to quantify above- and belowground allocation and transfer of carbon and nitrogen between 18 plant species. We measured isotopic enrichment of >1,800 leaves and roots, and found morphological plant? traits and tissue stoichiometry were the most important predictors of interspecific resource transfer. Labeled “donor” plants assimilated isotopic tracers at similar rates; however, nitrogen was preferentially transferred to annual and forb “receiver” plants compared to perennial and grass receiver plants due to differences in tissue stoichiometry. Our findings point to a simple mechanistic answer for long-standing questions regarding mutualism and transfer of resources between plants via mycorrhizal networks.