FOLIAR UPTAKE OF WET-DEPOSITED NITROGEN BY NORWAY SPRUCE - AN EXPERIMENT USING N-15

High rates of atmospheric nitrogen deposition have been shown to cause forest decline in some areas. A number of soil-mediated damage mechan isms are well-characterised but much less is known about the role of d irect uptake of N by the canopy. The stable N-15 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 n utrient status (P and K) on N uptake was also investigated. Simulated rain solutions containing 99 atom% (NH4+)-N-15-N or 99 atom% (NO3-)-N- 15-N at 15 mg N l(-1) were applied as a fine mist for 0.5 h. Woody twi gs, current needles and current +1 year needles were sampled before an d after misting and analysed for N-15 enrichment and total N by automa ted mass spectrometry. All three tissue types showed some capacity for uptake of both (NO3-)-N-15-N and (NH4+)-N-15-N, although in nitrate t reatments, N absorption was only statistically significant in woody tw igs. Uptake rates were significantly higher in twigs compared with nee dles and in ammonium treatments compared with nitrate treatments, but only in PK fertilised trees. The concentrations of P and K in spruce f oliage did not significantly affect the amount of N taken up by the ca nopy. Extrapolation of these data to mature forests suggests that foli ar 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 bran ches and twigs could potentially make an important contribution to tot al N requirements, although uptake may have been overestimated as it w as not possible to differentiate between biological uptake of N-15 and retention in the bark. (C) 1998 Elsevier Science Ltd. All rights rese rved.