Seasonal variation in nitrate reductase activity in needles of high-elevation red spruce trees

Abstract

To assess seasonal and site variation in foliar nitrate reductase activity and its utility as a biochemical marker for the uptake of nitrogen oxide pollutants in high-elevation forests, we measured nitrate reductase activity in current-year needles of red spruce (Picearubens Sarg.) saplings at two high-elevation stands (1935 and 1720 m) in the Great Smoky Mountains, North Carolina. Measurements spanned two growing seasons between September 1987 and September 1988. Nitrate reductase activity peaked near 60 nmol•g−1•h−1 at both sites in September and October 1987 and August 1988 and declined 80% in November 1987 and 65% in September 1988. Although nitrate reductase activity was 30% greater in saplings at the higher site relative to the lower site in September and October 1987, activity dropped to approximately 10 nmol•g−1•h−1 at both sites in November 1987. No differences among sites were evident the following year. Comparing deposition of nitric acid vapor at a nearby site to nitrate reductase activity suggests that needle nitrate reductase activity is not an unequivocal marker for foliar uptake of nitrogen oxides during air pollutant episodes. The changes in soil nitrate levels in this system provide preliminary evidence that foliar nitrate assimilation may, in part, include nitrate taken up from the soil, as the highest activity occurred during periods of higher A-horizon nitrate concentrations in 1988. These measurements of nitrate reductase activity suggest that red spruce are capable of assimilating nitrate in foliage in the field and that the nitrate assimilation capacity varies throughout the year.