Stem radial growth of host trees is not sustained after thirteen years of nitrogen addition but the abundance of high-biomass ectomycorrhizal fungi doubles

Justine Karst, Joshua Wasyliw, James Franklin, Scott X. Chang and Nadir Erbilgin, Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada, Joseph D. Birch, Renewable Resources, University of Alberta, Edmonton, AB, Canada

Background/Question/Methods
Nitrogen (N) availability in boreal forests has been altered by aspects of global change including climate change and aerial deposition of N. Ectomycorrhizal (EM) fungi mediate nutrient uptake of boreal trees, receive considerable amounts of photosynthates from plant partners, and influence soil organic matter dynamics. Thus, shifts in their abundance and community composition are important to consider in the response of forests to N availability. Importantly, ectomycorrhizas are symbiotic, so the response of EM fungi to N cannot be understood in isolation of their plant partners. Most previous studies, however, neglect to measure the response of host trees to N addition simultaneous with that of fungal communities. In addition to being one-sided, most of these studies have also been conducted in coniferous forests or plantations. Deciduous and ‘dual-mycorrhizal’ tree species, namely those that form ecto- and arbuscular mycorrhizas, have received little attention despite being widespread in the boreal forest. We hypothesized that aspen (Populus tremuloides), a dual-mycorrhizal tree species, would increase in stem radial growth with N addition. Informed by results of studies mainly in coniferous stands, we hypothesized that an increase in inorganic N would reduce EM fungal biomass, change the composition of EM and arbuscular mycorrhizal (AM) fungal communities, and increase root colonization by AM fungi. We applied N (30 kg ha-1 year-1; n = 4) for thirteen years to aspen-dominated stands and measured stem growth by annual ring width, scored their roots for the presence of high- (i.e., extramatrical hyphae present) and low- (extramatrical hyphae absent) biomass ectomycorrhizas, measured root colonization by EM and AM fungi, and characterized mycorrhizal fungal communities using Illumina MiSeq.

Results/Conclusions
Nitrogen addition initially increased stem radial growth of aspen but it was not sustained at the time we characterized their mycorrhizas. After thirteen years, stem radial growth had converged with that of control plots, and the abundance of high-biomass EM fungi doubled. No changes occurred in EM and AM fungal community composition, or their abundance measured as root colonization. This dual-mycorrhizal tree species did not shift away from EM fungal dominance with long-term N input. The unexpected increase in high-biomass EM fungi with N addition may be due to increased carbon allocation by trees to fungal partners. Given the focus on conifers in past studies, a broader view on N dynamics in boreal forests may be necessary when predicting their response to changes in N availability.