UPTAKE OF AMMONIUM AND NITRATE IONS FROM ACID MIST APPLIED TO SITKA SPRUCE [PICEA-SITCHENSIS (BONG) CARR] GRAFTS OVER THE COURSE OF ONE GROWING-SEASON

Cloned Sitka spruce grafts were subjected to NH4NO3 mist (1 mm day(-1) equivalent) acidified with H2SO4, for 4 days a week, from April until November. Three N treatments at pH 5.0 used 0.01 mol m(-3) (NH4)-N-15 (NO3)-N-15, 1.6 mol m(-3) (NH4)-N-15 NO3 or 1.6 mol m(-3) NH4 (NO3)-N -15. At pH 2.5, the treatments were (NH4)-N-15 (NO3)-N-15, (NH4NO3)-N- 15 and NH4 (NO3)-N-15 all at 1.6 mol m(-3). At the end of the growing season, N-15 was found in every part of the trees, even when NH4NO3 wa s supplied at 0.01 mol m(-3). It was shown that both NH4+ and NO3- cou ld enter needles to a similar degree and be transported to the roots. No differences between pH treatments or clones could be demonstrated. Entry of labelled N via the roots, in those trees with unprotected com post, was reflected in higher fine root N-15 content, but no change in shoot N-15 levels. Per cent incorporation expressed as labelled N as a percentage of total N, was generally at its highest in bark, and in older needles in all treatments, values ranging from 5 to 11%. When N- 15 content was expressed as total content for each tree part on a per tree basis, 25-36% was found in current year needles, with a further 1 2% in current year bark. The most reproducible data was that for dry w eight of tree parts as per cent of the whole tree, where proportions c ompared closely between treatments and the two clones used. The implic ations of the results for cuticular transport mechanisms, N storage an d internal cycling are discussed.