COMPOSITION OF ORGANIC-N AND N-15-LABELING UNIFORMITY IN ALDER LEAVES

The composition of organic-N in alder leaves (Alnus cordata and A. inc ana) fertilized with N-15-labelled KNO3 was studied by a two-step hydr olysis with 1N and 3N hydrochloric acid. The distribution of organic-N (NH4+-N, amide-N, hexosamine-N, amino acid-N, hydrolyzable unidentifi ed-N and non-hydrolyzable-N) did not exhibit significant differences b etween the two species. Of the total N 80% was hydrolyzable, with 55-6 1% being extracted in the first hydrolysis (HCl 1N x 3 h) and 19-24% i n the second (HCl 3N x 3 h). The major fraction was amino acid-N (41%) , followed by hydrolyzable unidentified-N (31%), amide-N (5%), hexosam ine-N (1-2%) and ammonium-N (<1%). The isotopic abundance of N-15 in s ome fractions differed from N-15 values of total leaf organic-N (P < 0 .05 to P < 0.01), suggesting that the leaf material was not uniformly labelled. Nevertheless, these differences were less than 3% in the maj or fractions (amino acids, hydrolyzable unidentified-N and non-hydroly zable organic-N), which accounted for more than 92% of leaf N. This la st result, and the fact that in both species N-15 distribution did not differ significantly from N-14 distribution, suggests that labelled a lder leaves obtained by this method are acceptable for decomposition s tudies or for leaf N availability experiments.