ATMOSPHERIC AMMONIA - ISSUES ON TRANSPORT AND NITROGEN ISOTOPE MEASUREMENT

Isotopes of nitrogen (N-15) have been used to evaluate N transport in soil-plant systems, but these studies generally ignore the atmospheric component of N balance. Recent studies of atmospheric ammonia (NH3) t ransport have shown the gaseous N component can be significant due to emission and absorption exchanges with the atmosphere. The purpose of this paper is to review measurements of atmospheric N cycling made by ourselves and others, and investigate how atmospheric transport may in fluence the conclusions of isotopic N studies. Soil and plant N transp ort were studied using N-15 while simultaneously measuring net atmosph eric NH3 transport using micrometeorological techniques. Simultaneous N-15 and micrometeorological studies have shown significant gaseous NH 3 losses from soils and plants as well as the potential for significan t NH3 absorption. These measurements have shown N transport measured b y the two techniques to agree closely when there was no plant activity (during drought). With plant activity, and the associated substitutio n of N-15 in the plant by N-14 from atmospheric NH3, NH3 losses measur ed by N-15 were 2 to 6 times larger than net NH3 losses measured by mi crometeorological techniques. Although N-15 studies are valuable for c omparison of treatments, caution must be exercised in the use of isoto pes where isotope exchange between the plant and atmosphere has not be en taken into consideration. (C) 1998 Elsevier Science Ltd. All rights reserved.