1. To distinguish among hypotheses on the importance of resource-exchange ratios in outcomes of mutualisms, we measured resource (carbon (C), nitrogen (N), and phosphorus (P)) transfers, and their ratios, between Pinus taeda seedlings and two ectomycorrhizal (EM) fungal species, Rhizopogon roseolus and Pisolithus arhizus in a laboratory experiment.

2. We evaluated how ambient light affected those resource fluxes and ratios over 3 time periods (10, 20, and 30 weeks), and the consequences for plant and fungal biomass accrual, in environmental chambers.

3. Our results suggest that light availability is an important factor driving absolute fluxes of N, P, and C, but not exchange ratios, although its effects vary among EM fungal species. Declines in N:C and P:C exchange ratios over time, as soil nutrient availability likely declined, were consistent with predictions of biological market models. Absolute transfer of P was an important predictor of both plant and fungal biomass, consistent with the excess resource exchange hypothesis, and N transfer to plants was positively associated with fungal biomass.

4. Altogether, light effects on resource fluxes indicated mixed support for various theoretical frameworks, while results on biomass accrual better supported the excess resource exchange hypothesis, although among-species variability is in need of further characterization.

We measured resource (carbon (C), nitrogen (N), and phosphorus (P)) transfers, and their ratios, between Pinus taeda seedlings and two ectomycorrhizal (EM) fungal species, Rhizopogon roseolus and Pisolithus arhizus in a laboratory experiment. We evaluated how ambient light affected those resource fluxes and ratios over 3 time periods (10, 20, and 30 weeks), and the consequences for plant and fungal biomass accrual, in environmental chambers. Plants and fungi were grown in chambers with a mesh barrier in the soil, which allowed only fungi (and not roots) to grow into a separate fungus-only chamber where fungal respiration rates could be measured; respiration rates were integrated over the different growth periods to estimate total amounts of C respired in each growth period. Ergosterol analyses were used to estimate fungal biomass. These methods allowed C transferred to fungi to be separated into a component respired and a component incorporated into fungal biomass. To test effects of light, fungal species, and time on absolute amounts and ratios of transferred resources (Question 1), data on C, N, and P fluxes, and N:C and P:C exchange prices were analyzed as response variables in separate univariate analyses using linear mixed-effects models using the lmerTest package in R version 3.5.2, with growth period (1, 2, and 3), light level (high and low), EM fungal species, and their interactions as fixed effects. The flux of C was also partitioned out into separate variables of C in fungal biomass and C respired by fungi, as well as carbon use efficiency (CUE), calculated as C in fungal biomass divided by total C transferred to fungi. These variables were also analyzed as above, to further understand how experimental factors may have affected carbon partitioning in the system. To test for effects of resource transfer on accumulated fungal and plant biomass (Question 2), we conducted model selection among all possible linear mixed-effect models for which the candidate variables were the main effects of the five resource flux variables representing effects of total resource transfer and exchange ratios (C, P, N, N:C, N:P).