Incorporation studies of NH4+ during incubation of organic residues by N-15-CPMAS-NMR-spectroscopy

This study focuses on the processes occurring during incorporation of inorganic nitrogen into humic substances. Therefore rye grass, wheat straw, beech saw dust, sulphonated lignin and organosolve lignin were incubated together with highly N-15-enriched ammonium sulphate in the laboratory for 600 days. Samples from the incubates were periodically analysed for weight loss, and carbon and nitrogen contents. The samples were also analysed by solid-state C-13- and N-15-CPMAS-NMR-spectroscopy to follow the turnover of the materials during incubation. Most of the detectable N-15-signals was assigned to amide-peptide structures. The remaining intensities could be ascribed to free and alkylated amino groups, and those on the low field side of the broad amide-peptide signal to indole, pyrrole and nucleotide derivatives. Abiotic reactions of ammonia with suitable precursors and the formation of pyridine, pyrazine or phenyloxazone derivatives were not observed. Signals from ammonia and nitrate occurred only at the end of the incubation.