Atmospheric ammonia: Issues on transport and nitrogen isotope measurement

Isotopes of nitrogen (15N) 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) transport 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 influence the conclusions of isotopic N studies. Soil and plant N transport were studied using 15N while simultaneously measuring net atmospheric NH3 transport using micrometeorological techniques. Simultaneous 15N and micrometeorological studies have shown significant gaseous NH3 losses from soils and plants as well as the potential for significant NH3 absorption. These measurements have shown N transport measured by the two techniques to agree closely when there was no plant activity (during drought). With plant activity, and the associated substitution of 15N in the plant by 14N from atmospheric NH3, NH3 losses measured by 15N were 2 to 6 times larger than net NH3 losses measured by micrometeorological techniques. Although 15N studies are valuable for comparison of treatments, caution must be exercised in the use of isotopes where isotope exchange between the plant and atmosphere has not been taken into consideration.