Foliar uptake of wet-deposited nitrogen by norway spruce: an experiment using 15N

High rates of atmospheric nitrogen deposition have been shown to cause forest decline in some areas. A number of soil-mediated damage mechanisms are well-characterised but much less is known about the role of direct uptake of N by the canopy. The stable 15N isotope has been used in this experiment to quantify the assimilation of wet-deposited N by foliage and branches of 5 yr old Norway spruce trees. The effect of nutrient status (P and K) on N uptake was also investigated. Simulated rain solutions containing 99 atom% 15NH+4-N or 99 atom% 15NO-3-N at 15 mg N ℓ-1 were applied as a fine mist for 0.5 h. Woody twigs, current needles and current +1 year needles were sampled before and after misting and analysed for 15N enrichment and total N by automated mass spectrometry. All three tissue types showed some capacity for uptake of both 15NO-3-N and 15NH+4-N, although in nitrate treatments, N absorption was only statistically significant in woody twigs. Uptake rates were significantly higher in twigs compared with needles and in ammonium treatments compared with nitrate treatments, but only in PK fertilised trees. The concentrations of P and K in spruce foliage did not significantly affect the amount of N taken up by the canopy. Extrapolation of these data to mature forests suggests that foliar uptake is unlikely to exceed about 5% of annual N requirements, and will only make a small contribution to “N-saturation” of vegetation. The results of this experiment indicate that absorption of N by branches and twigs could potentially make an important contribution to total N requirements, although uptake may have been overestimated as it was not possible to differentiate between biological uptake of 15N and retention in the bark.