An Experimental Test of Controls on Resource Exchange in an Ectomycorrhizal Mutualism

Models of resource exchange mutualisms utilize economic principles to explain how the costs and benefits of these interactions vary with environmental context. All of these models use a ratio of resource exchange (e.g., nitrogen: carbon) as the central variable, and it is unclear whether such exchange ratios predict outcomes of mutualisms in natural systems. Corrêa et al. (2008) hypothesized instead that the absolute flux of the most limiting nutrient, rather than the ratio of the two exchanged resources, best explains the benefits of resource exchange mutualisms. To distinguish between these two competing hypotheses, we measured resource transfers, and their ratios, between Pinus taeda seedlings and two ectomycorrhizal (EM) fungal species, Rhizopogon roseolus and Pisolithus arhizus. We evaluated how carbon availability to plants (manipulated with high and low light exposure) affected those resource fluxes and ratios over 3 time periods (10, 20 and 30 weeks) using mycocosms in environmental chambers. Our results suggest that higher light availability increases resource exchange between mycorrhizal mutualists, and that N:C resource exchange ratios are higher under low light, but that limiting soil nutrients have a stronger effect on plant growth than resource exchange ratios. These results suggest that the “exchange ratio hypothesis,” and the “total flux hypothesis” are both correct in their predictions, implying that when mycorrhizal plants have additional C to trade to their mycorrhizal fungi, it has the potential to promote an increase in nutrients to the plant in return, changing the price of exchange without detriment to either mutualist