Nitrate reductase activity and foliar δ15N measurements as integrative indicators of foliar incorporation of atmospheric nitrogen pollution

Plant canopies are major sinks for atmospheric nitrogen pollution through several depositional pathways. Therefore, foliar nitrogen incorporation of gaseous reactive nitrogen could represent a considerable source of nitrogen to plants. However, plants are known to vary widely in their foliar assimilation capacity, and few studies have examined this variation among plant species at realistic nitrogen pollution concentrations. Using hydroponics-fumigation experiments, we examined patterns of growth, nitrate reductase activity (NRA) and foliar δ¹⁵N values in response to various fumigations of enriched gaseous ¹⁵NO₂ in seedlings of red maple (Acer rubrum) and yellow birch (Betula alleghaniensis) to quantify differences in foliar NO₂ assimilation. Soil solutions in the system were supplied with two levels of NO₃^- at a known isotopic composition. Both species incorporated up to 8% of their total assimilated nitrogen using the foliar uptake pathway under nitrogen-limited conditions. Leaf tissue contained the highest proportion of NO₂-N (3-12%) compared to root and stem tissues (1-5%) for both species. Foliar NO₂ incorporation corresponded with increased leaf NRA in both species. Yellow birch showed higher leaf activities and higher ratios of leaf NRA to root NRA than red maple under NO₂ exposure. Neither species showed a significant response in biomass production or allocation to NO₂ exposure. Results suggest that the foliar NO₂ incorporation contributes a significant proportion of nutritive nitrogen to plant metabolism, and integrative measurements of NRA and foliar δ¹⁵N values may be reliable indicators of foliar incorporation of atmospheric nitrogen pollution in terrestrial ecosystems.