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

Abstract

Plants and mycorrhizal fungi form mutualistic relationships that affect how resources flow between organisms and within ecosystems. Common mycorrhizal networks (CMNs) could facilitate preferential transfer of carbon and limiting nutrients, but this remains difficult to predict. Do CMNs favor fungal resource acquisition 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)? We used stable isotope tracers (13CO2 and 15NH3), plant traits, and mycorrhizal DNA to quantify above- and belowground carbon and nitrogen transfer between 18 plant species along a 520-km latitudinal gradient in the Pacific Northwest, USA. Plant functional type and tissue stoichiometry were the most important predictors of interspecific resource transfer. Of "donor" plants, 98% were 13C-enriched, but we detected transfer in only 2% of "receiver" plants. However, all donors were 15N-enriched and we detected transfer in 81% of receivers. Nitrogen was preferentially transferred to annuals (0.26 {+/-} 0.50 mg N per g leaf mass) compared to perennials (0.13 {+/-} 0.30 mg N per g leaf mass). This corresponded with tissue stoichiometry differences. Our findings suggest that plants and fungi that are located closer together in space and with stronger demand for resources over time are more likely to receive larger amounts of those limiting resources.