Uniformity of labelling of alder leaves fertilized with NH4+15N and NO3−15N by roots or leaves

Labelling of grey alder leaves following root fertilization (RF) and leaf fertilization (LF), with either NH4+15N (A) or NO3−15N (N) (10 atom% 15N abundance), was compared. Fifteen leaf organic-N fractions were obtained by a two-step hydrolysis (H1 = HCl 1 n × 3 h; H2 = HCl 3 n × 3 h) and labelling efficiency, uniformity and repeatability were studied. The efficiency of labelling, measured by the 15N abundance achieved in the leaves, decreased in the order: N-RF ≥ N-LF ≥ A-RF > A-LF. The most uniform labelling was found in the leaves of A-RF trees since only one organic-N fraction, that accounted for 0.7% of leaf N, had labelling values significantly different from total leaf N. In the leaves of N-RF trees labelling values significantly different from total leaf N were found in two organic-N fractions (0.8% of leaf-N). Labelling uniformity decreased in LF trees: A-LF led to significant differences in three organic-N fractions (4.4% of leaf N), whereas in N-LF trees there were significant differences in other three fractions (12.1% of leaf N). The lowest deviation in the isotopic abundance (DIA) between each organic-N fraction and the total leaf organic-N was obtained in leaves from A-RF trees (mean 0.8%). DIA values for leaves from N-RF trees (mean 1.7%) and A-LF trees (mean 1.9%) were also acceptable. The worst DIA values were those of the leaves from N-LF trees (mean 3.5%). Repeatability of the organic-N labelling values among the different seedlings of each treatment decreased in the order: A-RF (CV = 0.38%), N-RF (CV = 0.68%), A-LF (CV = 0.76%) and N-LF trees (CV = 1.51%). In conclusion, for both NH4+15N and NO3−15N fertilizers, RF gave better labelling efficiency, uniformity and repeatability than LF. Leaves of trees supplied with A-RF gave the best labelling uniformity and repeatability, with acceptable labelling values. Leaves of trees supplied with N-RF showed the highest labelling values and with acceptable labelling uniformity and repeatability.