Models of resource exchange mutualism utilize economic theories to explain how the costs and benefits of these interactions vary with context. All of these models utilize a ratio of resource exchange as the central variable (i.e. the flux of carbon (C) to a symbiont compared to the flux of nitrogen (N) or phosphorus (P) to the host), but measurement of this variable in natural systems, and how it varies with context, has been difficult to achieve. We measured resource exchange ratios between Pinus taeda seedlings and two ectomycorrhizal (ECM) fungal species, Rhizopogon sp. and Pisolithus paradoxus, and evaluated the context-dependency of those ratios (high and low light availability) over 28 weeks using mycocosms in environmental chambers. C:N and C:P exchange ratios were determined by comparing the amount of C respired by fungi and C assimilated in fungal biomass to the amounts of N and P received by plants and assimilated in plant tissues over three time periods.
Results/Conclusions
Resource exchange between symbionts increased over time as the mutualism developed, but exchange ratios remained similar across the two ECM fungal species. As assumed by all models of economic exchange, seedling growth rate was negatively correlated with both C:N and C:P ratios. Growth rate was more negatively correlated with C:N and C:P in low light than in high, confirming that plant growth relies directly on resource exchange rates. As predicted by economic models of resource exchange mutualism, across both fungal species, C:N and C:P ratios were higher for seedlings grown in a high light environment than those grown in a low light environment. The research here not only confirms key assumptions and predictions of all economic models of resource exchange, but also provides the first estimates of cumulative exchange rates and the context dependency of those rates.