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Foliar nitrogen isotope (δN) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δN. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δN in potted red maple ( Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δN values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δN, and mycorrhizae on foliar δN in adult stands of A. rubrum, American beech ( Fagus grandifolia), black birch ( Betula lenta), and red oak ( Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO) concentration explained 0, 69, 23, and 45 % of the variation in foliar δN in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δN. There was no correlation between foliar δC and foliar %N with increasing atmospheric NO concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δN in several dominant species occurring in temperate forest ecosystems. [ABSTRACT FROM AUTHOR]