Potential canopy influences on the isotopic composition of nitrogen and sulphur in atmospheric deposition.

Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the N/N and S/S 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 having N/N or S/S 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 has N/N ratios higher than those in rainfall; (2) the N/N 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, S/S 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.