BIOGEOCIIEMISTRY OF AN OLD‐GROWTH FORESTED WATERSHED, OLYMPIC NATIONAL PARK, WASHINGTON1

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Abstract
The biogeochemistry of a coastal old‐growth forested watershed in Olympic National Park, Washington, was examined. Objectives were to determine: (1) concentrations of major cations and anions and dissolved organic C (DOC) in precipitation, throughfall, stemflow, soil solution and the stream; (2) nutrient input/output budgets; and (3) nutrient retention mechanisms in the watershed. Stemilow was more acidic (pH 4.0–4.5) than throughfall (pH 5.1) and precipitation (pH 5.3). Organic acids were important contributors to acidity in throughfall and stemflow and tree species influenced pH. Soil solution pH averaged 6.2 at 40 cm depth. Stream pH was higher (7.6). Sodium (54.0 μeq L‐1) and Cl (57.6 μeq L−1) were the dominant ions in precipitation, reflecting the close proximity to the ocean. Throughfall and stemflow were generally enriched in cations, especially K. Cation concentrations in soil solutions were generally less than those in stemilow. Ion concentrations increased in the stream. Dominant ions were Ca (759.7 μeq L−1), Na (174.4 μeq L−1), HCO3 (592.0 μeq L−1), and SO4 (331.5 μeq L−1) with seasonal peaks in the fall. Bedrock weathering strongly influenced stream chemistry. Highest average NO3 concentrations were in the stream (5.2 μeq L−1) with seasonal peaks in the fall and lowest concentrations in the growing season. Nitrogen losses were similar to inputs; annual inputs were 4.8 kg/ha (not including fixation) and stream losses were 7.1 kg/ha. Despite the age and successional status of the forest, plant uptake is an important N retention mechanism in this watershed.