NITROGEN ISOTOPE COMPOSITION OF AMMONIUM AND NITRATE IN BULK PRECIPITATION AND FOREST THROUGHFALL

The stable nitrogen isotope composition of ammonium (NH4*) and nitrate (NO3) ions in bulk precipitation and forest throughfall was determined at Walker Branch watershed, a low elevation site (340 m) in northeastern Tennessee, and in cloud water from Whitetop Mountain, a high elevation site (1680 m) in southwestern Virginia. Tests of the method used indicated that sample processing did not introduce significant isotopic fractionation in measurements of nitrogen isotope composition. Mean δ15N values for NH4and NO3 -in solutions of artificial rain were within 0.3%o of the mean δl5N values of nitrogen compounds used to make the solutions. Over a one-year period on Walker Branch watershed, the measured mean (±SD) δ515N value for NH+ 4 in bulk precipitation and throughfall ( — 3.4 + 2.1%0) was significantly less than that for N03( + 2.3±2.4%0). The same relative pattern was observed in cloud water from Whitetop Mountain: mean (±SD) δ15N in cloud water NH+4was — 5.5±3.2%0;i.e., isotopically lighter than NO3 (+1.4±4.8%0). The isotopic composition of NH+4in bulk precipitation and throughfall on Walker Branch watershed and in cloud water at Whitetop Mountain was consistent with origination through the washout of atmospheric NH3. However, the origins of NO3(gaseous versus aerosol precursors) could not be clearly established from δ5N measurements. © 1992, Taylor & Francis Group, LLC. All rights reserved.