Isotopic analysis of dissolved organic nitrogen in soils

Resúmen

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 was