Use of Diffusion for Automated Nitrogen-15 Analysis of Soil Extracts

Studies to evaluate the use of diffusion for automated ^I5N analysis of inorganic N in soil extracts showed that serious error can arise from use of the Devarda's alloy recommended for steam distillations and that the error can be avoided by using a commercial product of higher purity. These studies showed that serious error can also arise when NH₃^--N is diffused following NH₄⁺-N and that separate diffusions should be performed for NH₄⁺-N and (NH₄⁺+ N0₃-)-N. Other work demonstrated that the plastic specimen containers employed for diffusion can be reused if acid-washed, that diffusions can be performed using either light or heavy MgO without ignition to decompose carbonate, and that labeled N0₂- is completely removed from soil extracts by treatment with sulfamic acid before diffusion. A comparison of ^l5N analyses by steam distillation and diffusion using extracts from two soils revealed better agreement for the soil having a lower content of organic matter. Substantial differences in analyses by the two techniques for the soil having a higher organic-matter content were attributed to enzymatic conversions of inorganic N during the 6-d diffusion period.