Soil fungal necromass in deciduous-dominated boreal forest after 13 years of inorganic nitrogen addition

Ectomycorrhizal (ECM) fungi comprise a large proportion of the living and dead microbial-derived soil carbon (C) pool in boreal forests. Because soil nitrogen (N) and C cycles are closely interlinked, shifts in N availability and subsequent effects on dead fungal mass ("necromass") may influence C storage in soils. Several mechanisms could underlie the balance of fungal necromass production and stabilization, including fungal morphological traits and physiological traits and biochemical interactions between roots and ECM fungi. We applied inorganic N (30 kg ha-1 year-1) for 13 years in a boreal forest dominated by Populus tremuloides Michx. and measured total fungal necromass concentrations and total C concentrations in organic and mineral soil. We also measured total fungal biomass concentrations in soil (representing changes in inputs), condensed tannin and chitin concentrations of mycorrhizal roots (representing changes in necromass stabilization), and the potential genetic capacity of the ECM fungal community to produce chitinases (indicating chitin degradation potential) and class II peroxidases (indicating polyphenol degradation potential). We detected little effect of long-term N addition on soil fungal necromass concentration. Long-term N addition did not have a detectable effect on soil C concentration, standing fungal biomass, fine root tannin or chitin concentrations. Despite downward trends, there was also no detectable effect of N addition on the potential genetic capacity of the ECM fungal community to produce chitinases or class II peroxidases. Our study indicates that 13 years of inorganic N addition in a deciduous broadleaf-dominated boreal forest has little detectable effect on soil fungal necromass concentrations or potential underlying mechanisms. However, morphological and physiological traits of the ECM community appear decoupled in response to inorganic N addition, representing key functional responses that warrant further investigation.Read the free Plain Language Summary for this article on the Journal blog.