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 [sup.15]N-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 [sup.15]N-DON signature is calculated from the difference between total dissolved [sup.15]N ([sup.15]N-TDN) and inorganic [sup.15]N. We quantified the [sup.15]N recovery and signature of DON, N[H.sub.4.sup.+], and N[O.sub.3.sup.-] in a series of inorganic N/DON mixtures (with a TDN concentration of 10 mg N [L.sup.-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 < 2.9%. Hence, reliable [sup.15]N-DON values are obtained over an DON concentration range of 2.3-10 mg [L.sup.-1]. High levels of DON could influence the accuracy of [sup.15]N-N[O.sub.3.sup.-] mainly at DON:N[O.sub.3.sup.-] ratios above 0.4. Evaluation of alternative N[O.sub.3] measurements is still necessary. Our method is applicable for soil solution samples and soil extracts and has no risk of cross-contamination. Potential applications are large, in particular for [sup.15]N tracer studies, and will increase our insight in DON behavior in soils. 10.1021/ac1018183