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

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.