Thirteen years of nitrogen addition increases ectomycorrhizal fungal biomass, does not alter its degradability, and stagnates growth of host trees

Nitrogen availability in the boreal forest has been disrupted by global change. As ectomycorrhizal fungi play critical ecosystem roles, shifts in their abundance and community composition must be considered in the response of forests to nitrogen disruptions. We applied nitrogen (30 kg N ha-1 year-1) for thirteen years to stands dominated by aspen and measured growth of trees and their fungal partners, community composition of ecto- and arbuscular mycorrhizal fungi, and processes influencing the degradability of soil fungi. The responses to nitrogen addition by deciduous, dual-mycorrhizal tree species and their fungal communities have rarely been tested. Though nitrogen addition initially increased growth of aspen, it was not sustained. When tree growth stagnated, the presence of high-biomass ectomycorrhizal fungi doubled. There were no differences in fungal community composition. Factors influencing degradability of soil fungi-tannins in aspen roots and chitin concentrations of ectomycorrhizal fungi colonizing those roots-did not change with nitrogen addition. The increase in high-biomass ectomycorrhizal fungi with nitrogen addition was likely a consequence of increased carbon allocation by host trees caused by growth-limitation. Given the narrow focus on conifers in past studies, a broader view on nitrogen dynamics in boreal forests may be necessary.