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 ?15N values for NH4and NO3 -in solutions of artificial rain were within 0.3 per cento of the mean ?l5N values of nitrogen compounds used to make the solutions. Over a one-year period on Walker Branch watershed, the measured mean (±SD)?515N value for NH+ 4 in bulk precipitation and throughfall ( - 3.4 + 2.1 per cent0)was significantly less than that for N03( + 2.3±2.4 per cent0). The same relative pattern was observed in cloud water from Whitetop Mountain: mean (±SD)?15N in cloud water NH+4was - 5.5±3.2 per cent0;i.e., isotopically lighter than NO3 (+1.4±4.8 per cent0). 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 ?5N measurements.