STOICHIOMETRY OF NITROGEN AND BASE CATIONS IN ATMOSPHERIC FED FORESTS OF THE OREGON COAST RANGE

Nitrogen (N) is a critical limiting nutrient that regulates plant productivity and the cycling of essential base cations in forests. Increases in N availability beyond the threshold of plant and ecosystem needs may drive non-linear biogeochemical changes that include excess nitrate leaching and base cation depletion from soils. While typically associated with polluted regions, such changes may also occur in unpolluted forests of the Pacific Northwest due to legacies of soil N enrichment from biological N fixation in red alder. As part of a forest growth study, we sampled 20 young Douglas-fir stands in the Oregon Coast Range, and found that surface soil calcium (Ca) and magnesium (Mg) concentrations were inversely related to N across a gradient from 0.15 to 1.1 soil %N. Strontium isotope ratios indicate that N-rich forests are decoupled from weathering, and obtain > 97% of base cation nutrition from marine sea-salt aerosols. However, high Ca:Mg stoichiometry of plant demands relative to aerosol inputs selectively fosters Ca deficiency at high soil N. Plant and soil patterns were similar for sandstone versus basalt derived soils, indicating that biological N availability - not bedrock - controls coupled N and base cation cycling across areas of high N enrichment.