DOI: 10.5061/dryad.zkh1893d4
title: Dirt cheap: An experimental test of controls on resource exchange in an ectomycorrhizal symbiosis

description: 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.
description: 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).