Soil solution nitrogen concentrations and leaching rates as influenced by organic matter retention on a highly productive Douglas-fir site

Forest harvesting and organic matter management practices may affect the cycling of nutrients, particularly nitrogen (N), through the removal of different organic matter components (e.g. branches, foliage, coarse woody debris) from a site, and the associated changes in carbon-source quantity and quality. This study examined the influence of intensive harvesting and organic matter retention practices on soil N dynamics on a highly productive (site index I-II+) coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) site in southwestern Washington, while controlling for all competing vegetation. The specific focus of this investigation was to determine the effects of bole-only (BO) harvesting and total-tree harvesting plus coarse woody debris removal (TP) on soil solution N concentrations and leaching rates (to a depth of 1.0 m) during the third through fifth years following harvest. Additional comparisons were made between the harvested treatments and adjacent non-harvested portions of the same forest stand (FS). Soil solution monitoring over 3 years indicated that the increased organic matter retention associated with BO harvesting increased the total N concentrations and leaching flux to a depth of 1.0 m by roughly three times relative to TP harvest treatments. Nitrate comprised a majority of the 75, 29 and 4.5 kg ha−1 yr−1 of total N leached in the BO, TP and FS observations, respectively. The cumulative quantity of N leached to a depth of 1.0 m over 3 years was a small percentage (BO = 1.5%, TP = 0.6% and FS < 0.1%) of the 15 mg ha−1 total mineral soil N pool to a similar depth. In the two different organic matter retention treatments, this cumulative N flux was proportional (∼30%) to the amount of N left on site in the form of forest floor and coarse woody debris.