LAUSR

Quantifying the fate of organic nitrogen in aquatic systems is important to improve understanding of its recycling efficiency and long‐term preservation. The fate of organic nitrogen can be investigated with 15N labeling techniques, but relative amounts of 15N in different chemical forms are difficult to quantify. We present a “streamlined” method by combining Ammonium Retention Time Shift‐High Performance Liquid Chromatography with zinc reduction, and UV oxidation. This method does not require a pre‐isolation step of different forms of nitrogen from the sample. At a sample volume of 50 mL, and a total N concentration in the range of 0.5–40 μmol N L−1, and an 15N atom% of 20–80%, 15N concentrations for all N forms can be measured with this streamlined method, with a precision of within ±7%, and an accuracy of over 97%. We applied the method to investigating the short‐term fates of 15N during the degradation of 15N‐labeled amino acid and peptide. Recovery rates ranged from 93% to 110%, with an average of 102 ± 1.94%. As spiked 15N labeled alanine and/or peptide (Ala‐Val‐Phe‐Val) disappeared during sample incubations, a large fraction (ca. 13–66%) of the 15N was progressively transformed to non‐amino acid or non‐peptide dissolved organic nitrogen. This streamlined method offers quantitative estimates of potential fates of labile organic N compounds added to water samples containing in situ microbial consortia, and helps fulfill knowledge gaps in building the budget of N transformations of labile amino acids and peptides in aquatic systems.