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SummaryPlants and mycorrhizal fungi form close 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 preference remains difficult to predict. 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)? We used stable isotope tracers (13C and15N), plant traits, and fungal DNA, to quantify above- and belowground allocation and transfer of carbon and nitrogen between 18 plant species in restored prairie and introduced pasture systems in the Pacific Northwest, USA. We found that related plant species and connectivity with fungal communities did not predict resource flow. Instead, plant functional type 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. This corresponded tissue stoichiometry differences. Our findings point to a simple mechanistic answer for long-standing questions regarding mutualism and transfer of resources between plants via mycorrhizal networks.

Arbuscular mycorrhizal (AM) symbioses between plants and fungi are essential to the functioning of terrestrial ecosystems through maintaining soil stability, controlling nutrient cycles (e.g. C, N, P and K), and influencing competitive dynamics in plant communities. Despite the importance of AM symbioses, the ecological coassembly patterns of AM fungi-plant partners are not well characterized across environmental gradients. Further, it is unclear whether fungi forming associations with several plants of the same or different species – forming common mycorrhizal networks (CMNs) – preferentially allocate limiting resources within natural plant communities at the local-scale. We used an experimental prairie-pasture grassland system in three sites along a latitudinal gradient ranging from cool/wet to warm/dry climates to investigate how environmental conditions, local plant diversity and drought shift AM fungal composition and plant-fungal coassembly patterns across spatial scales. We show that plant-AM fungal assembly patterns are hierarchically structured, with environmental variables driving differences in AM fungal communities at the largest spatial scale (across sites), and plant host identity and diversity governing AM assembly at the local scale (within plot). Bipartite interaction networks revealed evidence for preferential partner selection between plants and fungi, while there was no evidence for nested assembly of plant-fungal partners. At the plot-level, we applied stable isotopes (13C and15N) to illustrate CMN assembly and nutritional function. There was no significant correlation between increased resource transfer among plants in a plot that shared more AM fungal partners; however, we identified specific AM fungi that were indicator taxa for increased plant isotope enrichment. Further research integrating stable isotope probing of fungal DNA in plant roots is necessary to more clearly illustrate the form and function of CMNs in grasslands under different environmental and plant diversity conditions.