A new method for using 18O to trace ozone deposition

Abstract

Isotopically labelled ozone (18O3) is an ideal tool to study the deposition of O3 to plants and soil, but no studies have made use of it due to the technical difficulties in producing isotopically enriched ozone. For 18O3 to be used in fumigation experiments, it has to be purified and stored safely prior to fumigations, to ensure that the label is present predominantly in the form ofO3, and to make efficient use of isotopically highly enriched oxygen. We present a simple apparatus that allows for the safe generation, purification, storage, and release of 18O3. Following the purification and release of O3, about half (by volume) of the 18O is present in the form of O3. This means that for a given release of 18O3 into the fumigation system, a roughly identical volume of 18O2 is released. However, the small volume of this concurrent 18O2 release (100 nmol molS1 in our experiment) results in only a minor shift of the much larger atmospheric oxygen pool, with no detectable consequence for the isotopic enrichment of either soil or plant materials. We demonstrate here the feasibility of using 18O as an isotopic tracer inO3 fumigations by exposing dry soil to 100 nmol molS1 18O3 for periods ranging from 1 to 11 h. The 18O tracer accumulation in soil samples is measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS), and the results show a linear increase in 18O/16O isotope ratio over time, with significant differences detectable after 1 h of exposure. The apparatus is adapted for use with fumigation chambers sustaining flow rates of 1m3 minS1 for up to 12 h, but simple modifications now allow larger quantities of O3 to be stored and continuously released (e.g. for use with open-top chambers or FACE facilities).