title: Potential canopy influences on the isotopic composition of nitrogen and sulphur in atmospheric deposition.
abstract: Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the15N/14N and34S/32S ratios of nitrate, ammonium and sulphate in rainfall. For systems with well-developed plant canopies, however, inputs of these ions from dry deposition or particulates may be more important than rainfall. The manner in which isotopic fractionation between ions and gases may lead to dry deposition and particulates having15N/14N or34S/32S ratios different from those of rainfall is considered. Data for rainfall and throughfall in coniferous plantations are then discussed, and suggest that: (1) in line with expectations, nitrate washed from the canopy has15N/14N ratios higher than those in rainfall; (2) the15N/14N ratios of ammonium washed from the canopy are variable, with high ratios being found for canopies of higher pH in conditions of elevated ambient ammonia gas concentrations; and (3) in accord with expectations and previous work,34S/32S ratios of sulphate washed from the canopy are not substantially different from those in rainfall. The study suggests that if atmospheric inputs are relevant to isotopic studies of the sources of nitrogen for canopied systems, then confident interpretation will require analysis of these inputs.
authors.affiliation: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK, , , , , , GB.
authors.forename: T H E
authors.initials: TH
authors.lastname: Heaton
authors.affiliation: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK, , , , , , GB.
authors.forename: Baruch
authors.initials: B
authors.lastname: Spiro
authors.affiliation: Institute of Terrestrial Ecology, Merlewood, Grange-over-Sands, Cumbria LA11 6JU, UK, , , , , , GB.
authors.forename: S Madeline C
authors.initials: SM
authors.lastname: Robertson
keywords: 15N/14N 34S/32S ratios; Atmosphere; Key words Nitrogen sulphur isotopes; Trees