Nutrient Dynamics Associated with Field-Drying of Harvest Residues On Clearcut Sites in Northwestern Ontario.

Increasing pressures to find inexpensive and renewable energy has turned attention to the utilization of forest harvesting residues as a supplemental fuel source. This increased utilization raises two concerns: 1) leaves, twigs, and branches, are very high in nutrient concentration and continued removal of such material may negatively affect long-term site productivity, and 2) on average, 50-60% of this material`s green weight is water, making transportation and burning costs much higher. There is great incentive to reduce both the moisture and nutrient content of the green biomass before transporting off site. Moisture content of residues left on site to field-dry can decrease by 50% in one year. Field-drying has also been found to leave nutrients on site by mechanical mass-loss, decomposition, and leaching. A one-year research project was carried out to investigate the effects of field-drying on the release of nutrients from black spruce, jack pine, and trembling aspen harvesting residues on clearcut sites in northwestern Ontario. Research objectives include determining: 1) overall nutrient contribution by shedding of foliage, nutrient leaching, and decomposition, and 2) influences of release rates by residue type (species and component) and microsite conditions (soil moisture and temperature). Early nutrient concentration data suggest that nutrients are in fact leached from the material immediately following harvest; this is especially evident with the more soluble nutrients, such as potassium. Moisture content decreased from 50% to as low as 10% in the first 4 weeks. The material, however, began to regain moisture and in some cases (trembling aspen coarse branch material) increased back up to 40% by week 16. Black spruce was found to shed most of its foliage within 4-8 weeks of drying, indicating that the shedding of foliage and fine branches during the drying process is likely the main contributing factor of nutrient return to the site.