276-7 Nitrogen Leaching Following Whole-Tree and Bole-Only Harvests on Two Contrasting Pacific Northwest Sites.

Poster Number 324

See more from this Division: S07 Forest, Range & Wildland Soils
See more from this Session: Management Effects In Forest Range and Wildland Soils: II
Tuesday, October 18, 2011
Henry Gonzalez Convention Center, Hall C
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Paul Footen1, Warren Devine2, Brian D. Strahm3, Rob B. Harrison1, Thomas Terry4 and Tim Harrington5, (1)School of Forest Resources, University of Washington, Seattle, WA
(2)USDA Forest Service, Olympia, WA
(3)Virginia Tech, Blacksburg, VA
(4)Weyerhaeuser Corporation (retired) and USDA Forest Service Pacific Northwest Research Station, Olympia, WA
(5)Pacific Northwest National Lab., Olympia, WA
Short-term pulses of increased N leaching typically follow the harvest of forest stands, but the magnitude of these pulses after whole-tree (WT) and conventional bole-only (BO) harvests remain difficult to predict on many sites. In this study we measured leaching for up to 6 and 8 years post-harvest on two Douglas-fir sites: a high productivity site with a silt loam soil and a low productivity site with a loamy sand soil. Whole-tree and bole-only harvest treatments were compared on each site, and presence/absence of 5 years of vegetation control (VC) was tested at one site. The magnitude and duration of post-harvest N-leaching pulses differed by site. At the high-productivity site, N leaching between years 3 and 8 post-harvest totaled 250 and 94 kg N ha-1 in BO and WT harvest treatments with VC, respectively. At the low-productivity site, N leaching totaled 32 and 17 kg N ha-1 from year 3 through year 6 post-harvest in BO and WT harvest treatments with VC, respectively. In both BO and WT treatments, annual N leaching did not fall below 5 kg N ha-1 until year 7 at the high-productivity site and until years 5 (BO) and 4 (WT) at the low-productivity site. In unharvested stands at both sites, annual N leaching was 2 kg N ha-1 or less each year. Cumulative amounts of N leached among site/treatment combinations were small compared to the soil total N pool (0.5 to 2.5%). The N leaching patterns among treatments at these two sites suggest that differences in soil C:N ratio (i.e., N status), the amount of post-harvest vegetation regrowth, and the N content of harvest residues influenced the amount of N leached. Although the maximum post-harvest N leaching pulse of 250 kg N ha-1 was much higher than those reported in most comparable studies, this level of N leaching is unlikely to be reached under operational conditions, where VC is less intensive.
See more from this Division: S07 Forest, Range & Wildland Soils
See more from this Session: Management Effects In Forest Range and Wildland Soils: II