Biomass Harvesting and Future Forest Productivity: The Significance of Fungal Import of Nitrogen Into Woody Residue.

Translocation of nitrogen (N) through mycelial cords of saprotrophic fungi is thought to be the mechanism responsible for observed increase in absolute N concentration in woody debris over time. It has been suggested that this mechanism may be responsible for storage of site N in woody debris after harvest. Woody debris removal from forests is increasing globally in response to escalating demand for bioenergy products. However, the relative importance of N storage in woody debris via saprotrophic fungi remains unclear. Mycelial-mediated translocation of nutrients to woody debris has been demonstrated for phosphorus, but little work has been done with N. Thus, this research will quantify the rate of N transfer into simulated woody debris (wood blocks) by the common saprotrophic fungus Hypholoma fasciculare. Given the heterogeneity of available N in forest soils, four nitrogen sources, enriched in ¹⁵N to enable tracing, inorganic (¹⁵NH₄⁺, ¹⁵NO₃^-), organic (¹⁵N-glycine) and litter- derived ¹⁵N will be supplied separately to mycelial cords growing from wood blocks in soil microcosms. An internal divider between the wood block and N source in the microcosms will ensure translocation can only be via mycelia cords. Two covariates, hyphal coverage and relative wood density, will be used to explain expected variability between experimental units. Three sampling times (d 1, 15 and 30) will be used to establish a rate of transfer for each N-form from soil into woody debris. These rates will then be used to establish an upper limit of N-transfer into wood blocks and will further be used to make inferences about the upper limit of N storage in woody debris after harvest. This research will advance our understanding of fungal N transfer and will produce further evidence to guide woody debris management in forests.