Deciphering factors controlling decay and nitrogen accumulation in coarse wood debris of five tree species using 15N labeled wood disks

Tree species and climate are considered the most critical drivers of coarse woody debris decay but disentangling their relative importance has proven challenging. We conducted a field accelerated decomposition experiment using 15N labeled wooden disks to investigate factors controlling fungi colonization, wood decay, and N fluxes during coarse woody debris decay. Wooden disks were produced from the stem of mature trees of 5 representative species in Eastern Canada (black spruce, balsam fir, white birch, sugar maple, and beech). Tree stems were collected from plots surface-sprayed with enriched 15N over 17 years. We monitored ergosterol concentration (as a proxy of fungal biomass), biomass loss, and 15N signal of wood disks placed on 5 sites with contrasted soil and climate properties. Biomass loss over 3 years reached up to 80 % for angiosperms, equivalent to about 50–80 years of decomposition. We report that tree species was the main predictor of wood decay, explaining more than 30 % of the variance, as well as fungal biomass and N content (up to 50 %). Climate and site parameters explained only 12 % of the variance in decay rates, 15 % of fungal biomass, and 31 % of N content. Finally, estimates indicate that fungal N transfer and biological nitrogen fixation account for a maximum of 30 % and 2 % of N inputs, respectively. This suggests that, in the studied sites, N derived from biological activity (i.e., fungal transfer and biological nitrogen fixation) is a minor source of exogenous N compared to abiotic sources (i.e., deposition, lateral transfers).