Nitrogen Isotope Composition of Ammonium and Nitrate in Bulk Precipitation and Forest Throughfall

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Abstract
The stable nitrogen isotope composition of ammonium (NH+ 4) and nitrate (NO− 3) 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 NH+ 4 and NO− 3 in solutions of artificial rain were within 0.3%o of the mean δ15N values of nitrogen compounds used to make the solutions. Over a one-year period on Walker Branch watershed, the measured mean (± SD) δ15N value for NH+ 4 in bulk precipitation and throughfall (− 3.4 ± 2.1%o) was significantly less than that for NO− 3(+ 2.3 ± 2.4%o). The same relative pattern was observed in cloud water from Whitetop Mountain: mean (± SD) δ15N in cloud water NH+ 4 was −5.5 ± 3.2%o; i.e., isotopically lighter than NO− 3(+ 1.4±4.8%o). The isotopic composition of NH+ 4 in 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 NO− 3 (gaseous versus aerosol precursors) could not be clearly established from δ15N measurements.