Nitrogen translocation and accumulation by a cord-forming fungus (Hypholoma fasciculare) into simulated woody debris

•Decay fungi translocated diverse forms of 15N from soil to their wood inocula.•211% more N accumulated in inoculated wood blocks vs. un-inoculated after 78 days.•The majority of accumulated N was acquired from underneath wood blocks.•Woody debris will accumulate more N if in close contact with the forest floor.

Nitrogen (N) accumulation into woody debris is well documented, but the forms of N translocated, the mechanism through which this occurs (purportedly via translocation by cord-forming fungi) and the temporal pattern of N accumulation are not well studied. In soil microcosms, we incubated wood blocks (Pseudotsuga menziesii) inoculated with the cord-forming decay fungus Hypholoma fasciculare  , and applied four forms of isotopically-labeled N (15NH4+, 15NO3-, 15N-glycine and 15N labeled litter) at the mycelial growing fronts to evaluate translocation and accumulation of diverse N sources by wood-decay fungi. Identically treated un-inoculated wood blocks were incubated. Three sampling times (6, 18 and 30 days after N addition) were used to identify trends in 15N transfer and total N accumulation. Wood blocks inoculated with H. fasciculare assimilated significantly more 15N than un-inoculated blocks for all 15N treatments on at least one sampling occasion. After a total of 73 days of incubation in microcosms, inoculated wood blocks increased in absolute N content by 211% relative to control blocks, but 80% of this accumulated N was lost by day 97. The small amount of 15N that was transferred into wood contrasted with the large increase in total N, suggesting that the site of N transfer was largely from soil under the wood blocks rather than at the site of 15N injection. The precipitous decline in N content was attributed to visible indications of mycelial senescence, possibly due to carbon limitation. If these findings are extrapolated to a cut-block, it follows that woody debris on the forest floor colonized by wood decay fungi would accumulate N, at least in the early stages of decay, and this process may immobilize N after harvest. If retention of N on site after harvest is a management objective, site preparation that maximizes accumulation of N in woody debris by saprotrophic fungi should be employed. This would involve homogenously distributing woody debris on site in close contact with the forest floor in order to promote fungal colonization and N accumulation.