Uptake and assimilation of atmospheric NO2 — N by spruce needles (Picea abies): A field study

NO₂ enters spruce needles by gas exchange through the slomata. Nitrate formed from NO₂ is reduced in the cytosol by nitrate reductase (NR), the rate limiting enzyme of the nitrogen assimilatory pathway. A linear relationship was found between the nitrate reductase activity (NRA), NO₂ concentration and the amount of N incorporated into amino acids and proteins, so that NRA was suggested as an estimate of NO₂-uptake. In the present field study, 50 spruce trees (Picea abies) have been selected, which grow in a natural habitat in a NO₂ concentration gradient in a forest crossed by a highway which is a major NO source. At part of the sites, the microclimatic conditions have been recorded, so that common models of local gas exchange of the needles could be used to estimate stomatal uptake of NO₂. NRA was investigated as a function of radiation and stomatal uptake on the day before needle sampling. Close to the highway NRA was permanently elevated with a maximum in summer. As with the laboratory results, a linear relationship between stomatal uptake and NRA was found. Total N — content of current year shoots was not affected by the additional N-source provided by airborne NO₂. The present study shows that the gas exchange models are consistent with the physiological reactions of spruce needles on a local level and therefore contribute to the validation of calculations of NO₂ dry deposition to spruce forests.