Stable nitrogen isotope ratios in wet and dry nitrate deposition collected with an artificial tree

Abstract

Amounts of dry NO₃-N deposition and N isotope ratios in wet and dry NO₃-N deposition have been simultaneously determined by examining differences between precipitation collected by open funnels and throughfall collected beneath an artificial Christmas tree. Samples were collected in a forest clearing on Walker Branch Watershed, near Oak Ridge, Tennessee. From mid-summer to early autumn, NO₃-N fluxes beneath the artificial tree were always greater than those measured in precipitation indicating the tree’s effectiveness as a passive collector of dry NO₃-N deposition. Dry NO₃-N deposition averaged 60 ± 9% of total (wet and dry) deposition. The mean (± SD) calculated δ15N value for NO₃-N in dry deposition was + 5.6 ± 2.1‰ (n= 6 sampling periods ranging from 4 to 15 days). On average, this was ≈ 6‰ heavier than measured δ15N values for NO₃-N in precipitation. The calculated δ15N value for NO₃-N in dry deposition was consistent with that expected if NO_x precursors to HNO₃ vapor (the major constituent of dry deposition at this site) originated principally from coal combustion.