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Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the 15 N/ 14 N and 34 S/ 32 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 15 N/ 14 N or 34 S/ 32 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 15 N/ 14 N ratios higher than those in rainfall; (2) the 15 N/ 14 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, 34 S/ 32 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.

Intra- and inter-tree variations in 13 C/ 12 C ratios were studied within a single clone plantation of 20-year-old Sitka spruce, some of which were treated with mist simulating acidic cloud water. For groups of trees of similar height and the same treatment, sampled at the same whorl height, δ 13 C values for current year needles showed variations (1 SD) of between 0.2 and 0.7‰. The variations reflect the seasonally averaged influences, on intercellular CO 2 concentrations, of slight variations in the microhabitat within a group. For a typical intra-group variation of 0.4‰ one may be able to distinguish between groups whose mean intercellular CO 2 concentrations differ by only 8 ppm. Acid misting resulted in a lowering of δ 13 C values by c. 0.7‰ (significant at the P≤0.05 level). This reflects higher intercellular CO 2 concentrations for acid misted trees, which can be interpreted in terms of their having assimilation rates c. 10% lower than those of control trees, and might explain the observed reduction in stem growth for acid-misted trees. Without careful attention to sampling strategy, however, these small inter-tree δ 13 C variations can be easily masked by the much larger intra-tree variations with height. Large gradients of increasing needle δ 13 C with height, of c. 0.5‰ m -1 , were observed in two untreated trees of different total height. The gradient was similar for both trees so, though δ 13 C values of both trees were identical close to their leaders (-27‰), the taller tree displayed much lower values close to the ground (-31‰). The gradients are believed to reflect lower light levels close to the ground, rather than the accumulation of respired CO 2 in the atmosphere. The different height response of stems versus needles, reflected by an increase in δ 13 C stems -δ 13 C needles with height (for cellulose), is discussed in terms of stem photosynthetic recapture of internally respired CO 2 .

Data are presented for the 15 N/ 14 N ratios of 140 indigenous terrestrial plants from a wide variety of natural habitats in South Africa and Namibia. Over much of the area, from high-rainfall mountains to arid deserts, the δ 15 N values of plants lie typically in the range -1 to +6‰; with no evident differences between C 3 plants and C 4 grasses. There is a slight correlation between δ 15 N and aridity, but this is less marked than the correlation between the δ 15 N values of animal bones and aridity. At coastal or saline sites, however, the mean δ 15 N values for plants are higher than those at nearby inland or non-saline sites-e.g.: arid Namib coast (10‰ higher than inland Namib); wet Natal beach (5‰ higher than inland Natal); saline soils 500 km from coast (4‰ higher than non-saline soils). High values were also found at one site where there were no marked coastal or saline influences. These environmental effects on the isotopic composition of plants will extend upwards to the animals and humans they support. They therefore have important consequences for the use of nitrogen isotope data in the study of the dietary habits and trophic structures of modern and prehistoric communities.