Potential canopy interception of nitrogen in the Pacific Northwest, USA

Nitrogen deposition is increasing worldwide from anthropogenic sources and encroaching upon previously N limited ecosystems. Recent literature reports increases in inorganic N deposition in Pacific Northwest forests of the United States due to expanding urbanization. We examined the contributions of atmospheric deposition of inorganic N to old-growth and second-growth Douglas-fir (Pseudotsuga menziesii) forests in the Cascade Mountains of southern Washington State. We used ion exchange resin lysimeters (IERs) to measure throughfall and compared it to data from a nearby atmospheric deposition recording station. Observed differences led us to install IERs above and throughout the canopy of an old- and second-growth forest stand at the Wind River Canopy Crane Research Facility. Total NH4-N and NO3-N deposition was 4.06 and 2.06 kg/ha, respectively, with NH4-N inputs varying seasonally. Canopy interception in the first 5 m of canopy was >80% of NO3-N deposition during the winter months, with negative net canopy exchange (NCE) accounting for nearly 90% of NO3-N input (uptake). NCE for NH4-N during the winter months was negligible. During the summer months, both NH4-N and NO3-N were taken up within the canopy. Contrary to the winter period, nearly all NH4-N entering the canopy was retained. Although the pattern of canopy interception varied according to canopy architecture and age of stand, all species were extremely efficient in reducing the input of inorganic N to the forest floor. Greater deposition in these stands as compared to the nearby NADP site was attributed to higher precipitation. Needle concentrations of N and δ15N showed no differences throughout the canopy profile, in contrast to both C and δ13C that exhibited significant increases from top to the lower levels. Differences in concentrations of C, δ13C, N and δ15N were noted among the old-growth species.