Aminoxidation of kraft lignin was carried out in a Parr reactor using (NH3)-N-15 as the main nitrogen source. Reaction parameters were set up until a total nitrogen content of approximate to 13 wt.% in lignin was achieved, in accordance with conditions of previous studies. Analytical tools such as FTIR, Py-GC/MS, and solid state NMR were used in this research. The nature of nitrogen bondings is discussed. The incorporation of the N-15 from ammoxidized lignin was followed in pumpkins (Zucchini cucurbita pepo L.) by means of N-15 emission spectroscopy. (c) 2005 Elsevier Ltd. All rights reserved.