Isotopic Analysis of Dissolved Organic Nitrogen in Soils

Abstract

Determination of the isotopic signature of dissolved organic nitrogen (DON) is important to assess its dynamics in terrestrial ecosystems. Analysis of (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 (15)N signature of DON in the presence of inorganic N using a persulfate digestion followed by microdiffusion. The (15)N-DON signature is calculated from the difference between total dissolved (15)N ((15)N-TDN) and inorganic (15)N. We quantified the (15)N recovery and signature of DON, NH(4)(+), and NO(3)(-) 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 (15)N-DON should be routinely possible; coefficient of variation was <2.9%. Hence, reliable (15)N-DON values are obtained over an DON concentration range of 2.3-10 mg L(-1). High levels of DON could influence the accuracy of (15)N-NO(3)(-) mainly at DON:NO(3)(-) ratios above 0.4. Evaluation of alternative NO(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 (15)N tracer studies, and will increase our insight in DON behavior in soils.