POTENTIAL CANOPY INFLUENCES ON THE ISOTOPIC COMPOSITION OF NITROGEN AND SULFUR IN ATMOSPHERIC DEPOSITION

Isotopic studies of nitrogen and sulphur ins puts to plant/soil system s commonly rely on limited published data for the N-15/N-14 and S-34/S -32 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. T he manner in which isotopic fractionation between ions and gases may l ead deposition and particulates having N-15/N-14 or ratios different f rom those of rainfall is considered. Data for rainfall and throughfall in coniferous plantations are then discussed, and suggest that: (1) i n line with expectations, nitrate washed from the canopy has N-15/N-14 ratios higher than those in rainfall; (2) the N-15/N-14 ratios of amm onium washed from the canopy are variable, with high ratios being foun d for canopies of higher pH in conditions of elevated ambient ammonia gas concentrations; and (3) in accord with expectations and previous w ork, S-34/S-32 ratios of sulphate washed from the canopy are not subst antially 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 requ ire analysis of these inputs.