PARENT SESSION
Oral Session #21: Nutrient cycling: Nitrogen.
Presiding: S. Hart
Monday, August 5. 1:00 PM to 3:45 PM. Grand Ballroom Central, Radisson.
Forest harvest, N saturation, and potential base cation depletion in coastal Oregon forests .
Perakis, Steven*,1,2, Bullen, Thomas3, Maguire, Doug2, Cromack, Kermit2, Waring, Richard2, 1 USGS Forest and Rangeland Ecosystem Science Center, Corvallis, OR2 Oregon State University, Corvallis, OR3 USGS Water Resources Division, Menlo Park, CA
ABSTRACT- Substantial accumulation and availability of soil N contributes to high productivity of conifer forests in the Oregon Coast Range. High soil N capital results primarily from long-term inputs of biologically fixed N from red alder, and soil biogeochemical changes can persist long after red alder replacement to conifer species. As a consequence, some coastal Oregon forests are not N limited, and appreciable watershed nitrate losses even suggest widespread N saturation. The potential for high tree productivity also facilitates intensive short-rotation forestry in this region, yet nutrient losses that accompany harvest cycles in these N-rich systems may impact long-term nutritional balances. Using soil and foliar data from 22 randomly selected Douglas-fir plantations throughout the Oregon Coast Range, we found that that surface soil exchangeable calcium and magnesium decreased strongly across a gradient from 0.15 to 1.1% soil N. We suggest a threshold above 0.4% soil N where nitrate leaching may contribute substantially to soil base cation depletion. Strontium isotope ratios measured in a 45 yr. old Douglas-fir forest indicate that these coastal ecosystems can obtain 70-95% of base cation nutrition from atmospheric sources, yet annual deposition provides only ca. 15% of calcium required for aboveground woody biomass production. These results raise the possibility that rapid tree growth, tree harvest, and associated base cation losses could deplete ecosystem calcium reserves. This may contribute to substantial reductions in forest growth and integrity that have occurred in coastal Oregon forests over the past decade.
KEY WORDS: nitrogen, base cation, strontium isotopes, Oregon