Carbon sequestration in Douglas-fir stands of the coastal coniferous forest region of Washington State

ADAMS, ALBERT; HARRISON, ROB; AMOROSO, MARIANO; BRIGGS, DAVE; COLLIER, RANDY; GONYEA, BOB; HASSELBERG, BERT; HAUKAAS, JOHN; O’SHEA, MEGAN

Olympia, Washington, USA

Otro; Productivity of Western Forests Conference: A Forest Products Focus; 2004

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station

Quantifying the effects of urea fertilization on carbon (C) in the solid and solution phases of soils can aid forest management. We evaluated the effect of urea fertilization on pure second-growth Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands growing in four soil series of the coastal coniferous forest region of Washington State. Our major objective was to determine the range of carbon flux and sequestration for this region. This paper covers our results for soil organic carbon (SOC) sequestration. Our soils were selected to give a range of texture from coarse-grained glacial to fine-textured volcanic. We compared soil types, soil depths and fertilization treatments over 20-year intervals. There was no difference in soil C in a coarse glacial outwash soil. By contrast, in a glacial sandy outwash soil all mineral horizons had more soil C and the forest floor less C compared with the paired unfertilized plot. In two volcanic soils our results provide evidence that DOC from the forest floor and A horizons was sequestered in both the epipedon and lower horizons. Differences between glacial and volcanic soils suggest that mechanisms of C were different in the coarse-textured materials than the fine-textured volcanic material. Urea fertilization added nitrogen to soils at all installations with differences ranging from 1-3 Mg N ha- 1. The 2 glacial soils had lower site indices. They had less SOC (mean of 87 versus 348 Mg ha- 1) but greater increases in aboveground C (mean increase of 41 versus 8 Mg ha- 1). Although fine-textured sites were more productive overall, the aboveground response to urea was limited in comparison to differences found in fertilized plots of the glacial sites. In contrast, SOC did increase with urea applications in fine-textured sites in excess of increases solely attributable to DOC.