Nitrogen leaching following whole-tree and bole-only harvests on two contrasting Pacific Northwest sites

Short-term pulses of increased N leaching typically follow the harvest of forest stands, but the magnitude of these pulses after conventional bole-only (BO) and whole-tree (WT) harvests often is difficult to predict. In this study, we measured N leaching until 6 and 8 years post-harvest on two western Washington Douglas-fir (Pseudotsuga menziesii var. menziesii) sites: a relatively low-productivity site with a gravelly loamy sand soil formed in glacial outwash and a high-productivity site with a silt loam soil derived primarily from basalt. We tested BO and WT harvest treatments and presence/absence of 5 years of competing vegetation control (VC). Trends in the magnitude and duration of post-harvest N-leaching pulses differed between sites. At the higher-productivity site, estimated N leaching between years 3 and 8 following harvest totaled 250 and 94 kg N ha−1 in BO and WT harvest treatments with VC, respectively. At the lower-productivity site, estimated N leaching totaled 32 and 17 kg N ha−1 between years 3 and 6 following harvest in BO and WT harvest treatments with VC, respectively. In both BO and WT treatments, annual N leaching did not fall below 2 kg N ha−1 (the rate measured in mature forest stands at both sites) until year 8 at the higher-productivity site and until year 6 at the lower-productivity site. Cumulative amounts of N leached among site/treatment combinations were small compared to the soil total-N pool and ranged from 0.5% to 2.5%. The N leaching patterns among treatments at these two sites suggest that differences in soil N content and C:N ratio, post-harvest vegetation regrowth, and harvest residues influenced the amount of N leached. The major source of leached N was most likely the soil (including forest floor) rather than harvest debris, based on site and treatment comparisons. Although the maximum post-harvest pulse of 250 kg N ha−1 was much higher than values reported in most comparable studies, this level of N leaching is unlikely to be reached under operational conditions, where VC is less intensive than in this study.

► Following harvest of forest stands, pulses of N leaching vary widely in magnitude.
► We measured N leaching for up to 6 and 8 years on two Pacific Northwest sites.
► N leaching was over 5-fold less at a low-productivity site than at a high-productivity site.
► N leaching was greater after bole-only harvest than after whole-tree harvest.
► Soil N was the likely source of the pulse of N leached at the high-productivity site.