Isotopic analysis of dissolved organic nitrogen in soils

Determination of the isotopic signature of dissolved organic nitrogen (DON) is important to assess its dynamics in terrestrial ecosystems. Analysis of 15N-DON, however, has been hindered by the lack of simple, reliable, and established methods. We evaluate three off-line techniques for measuring the 15N signature of DON in the presence of inorganic N using a persulfate digestion followed by microdiffusion. The 15N-DON signature is calculated from the difference between total dissolved 15N (15N-TDN) and inorganic 15N. We quantified the 15N recovery and signature of DON, NH4+, and NO3- in a series of inorganic N/DON mixtures (with a TDN concentration of 10 mg N L-1) for three lab protocols. Phenylalanine was used as a model compound for DON. The best lab protocol determined the concentration of inorganic N and TDN prior to diffusion using improved spectrophotometric techniques. An accuracy of 88% for 15N-DON should be routinely possible; coefficient of variation wa