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Atmospheric deposition of nitrogen and sulfur and preferential canopy consumption of nitrate in forests of the Pacific Northwest, USA

Fenn, Mark E., Ross, Christopher S., Schilling, Susan L., Baccus, William D., Larrabee, Michael A., Lofgren, Rebecca A.
Forest ecology and management 2013 v.302 pp. 240-253
national parks, ion exchange resins, canopy, nitrogen content, sulfur, monitoring, throughfall, nitrogen, dry deposition, wet deposition, nitrates, conservation areas, forests
Wet, dry and throughfall deposition of N and S were measured for 2years in three national parks in Washington State: Olympic, Mount Rainier, and North Cascades. Throughfall was measured using ion exchange resin (IER) collectors. A major objective of the study was to evaluate the effectiveness of IER throughfall measurements for monitoring deposition inputs, including cloudwater deposition, to forest stands in national parks and other protected areas. Wet deposition (0.9–2.0kgNha−1yr−1) and throughfall (0.5–1.2kgNha−1yr−1) deposition of inorganic N in the three parks were relatively low. Wet deposition of sulfur (1.0–3.2kgha−1yr−1) was similar to wet deposition of inorganic nitrogen except at OLYM where wet deposition of S was higher than for N because of marine sources of SO4–S. Throughfall N deposition was lower than wet deposition of N because of strong preferential canopy consumption of nitrate (NO3–N), particularly during the wet winter periods. This phenomenon was previously reported for forests in this region, but its apparent near ubiquity in the region had not been recognized. Data on preferential canopy retention of NO3–N from wet-deposited N is shown for 38 stands in the Pacific Northwest of which 21 are newly-reported data. Deposition of NO3–N in throughfall at MORA and NOCA was reduced by 87% and 93% compared to wet deposition over the 2years. In contrast, wet deposition of NH4–N was generally increased by passage through the canopy. This strong preferential canopy retention of wet-deposited NO3–N limits the usefulness of throughfall measurements as a N deposition monitoring approach in forests of the Pacific Northwest region of North America and in some other regions with low to moderate N deposition. As a potential remedy to this limitation, a simple method is proposed for estimating total N deposition in the study sites based on S/N ratios in wet deposition and throughfall S deposition.