Abstract:
Nutrient and particulate matter balances were established for five ecosystems dominated by Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] in the western Cascades, Oregon. Carriers of N and P were particulate matter (dust in precipitation and sediment in streams) and dissolved matter in both precipitation and stream water. The input and outflow of these materials (particulates, N and P) are interpreted in relation to atmospheric and forest ecosystem processes. Drainage basin sites are FC-2 (north), HJA-7, 9 and 10 (central), and CC-4 (south). Climate at all sites is maritime with cool-wet falls, winters, and springs and warm-dry summers. Soils, from igneous parent materials (Tuff, andesite, basalt), are porous, deep and of loam to silty clay texture. The basins are steep (12-70% slope) and completely covered by natural forest vegetation. varied by a factor of 2 or 3 over the range of runoff among the sites. The N outflow is explained by environmental factors, chemical composition f forest litter and the flushing efficiency of the annual runoff. Organic P may be controlled by litter quality, but ortho-P outflow was greatest where parent materials were dominantly tuff and declined according to the increasing proportions of less easily weathered basalt andesite, Dust depositions were 14. 3% of suspended sediment (SS) outflows, but varied according to the geologic erosion rate among sites. At FC-Z, dust deposition exceeded SS by 177%, but deposition was only 5. 0% of SS at CC-4. Dust deposition was an important source of soil material and partly offset geologic erosion. Mean N inputs, 1. 6-3. 2 times the outflows, were considered important in relation to known inputs by symbiotic fixation. Mean P inputs were 51 % of outflows, but were considered relatively unimportant because of the abundant supply in igneous bedrock.
