Arbuscular mycorrhizal and ectomycorrhizal fungi differ in the carbon cost of nutrient acquisition and growth benefit when grown with Populus grandidentata

Emel Kangi, Biology, West Virginia University, Morgantown, WV, Jonathan R. Cumming, Natural Sciences, University of Maryland Eastern Shore, Princess Anne, MD and Edward R. Brzostek, Department of Biology, West Virginia University, Morgantown, WV

Background/Question/Methods
Most terrestrial plants form symbioses with arbuscular mycorrhizal (AM) or ectomycorrhizal (ECM) fungi to exchange nutrients. While theoretical and predictive model frameworks posit that mycorrhizae reduce the C cost of nutrient acquisition, there are relatively few estimates of how the exchange rate of C for nutrients varies by mycorrhizal type and the estimates that do exist are confounded by plant species effects. Here, our objective was to investigate the extent to which the C cost of nutrient acquisition varies between AM and ECM fungi and across different levels of nutrient availability. To meet our objective, we used a single plant species, Populus grandidentata, which forms both associations, in order to minimize the influence of interspecific host differences and focus on the differences between AM (Rhizophagus intraradices) and ECM (Laccaria bicolor) fungi. We tested the effect of mycorrhiza type on P. grandidentata seedlings treated with low or sufficient N and low or sufficient P in a growth chamber experiment. To assay the C cost of nitrogen acquisition, we grew the seedlings in a C4-enriched corn soil to estimate new C3 inputs to the soil by leveraging shifts in the soil ∂13C. We also measured host biomass, %C, %N, %P, root-to-shoot ratio, and root colonization.
Results/Conclusions
Overall, we found that ECM fungi were most beneficial at low N levels and AM fungi were most beneficial at low P levels. At low N levels and sufficient P levels, seedlings with ECM fungi had a lower cost of nutrient acquisition and greater biomass than seedlings grown with AM fungi. By contrast, at low P levels, regardless of N availability, both colonization rates and the C cost of nutrient acquisition (i.e., the ratio of new C3 soil inputs to N and P gain by the plant) were lower for seedlings grown with AM fungi. Finally, at sufficient N and P, we found that plants that were non-mycorrhizal had lower C costs of nutrient acquisition and also greater biomass production. Collectively, our results show that the beneficial impacts of AM and ECM fungi are independent of plant species traits. Moreover, our data provides important constraints on parameter uncertainty in ecosystem models that are beginning to explicitly represent mycorrhizal processes.