USE OF DIFFUSION FOR AUTOMATED N-15 ANALYSIS OF SOIL EXTRACTS

Studies to evaluate the use of diffusion for automated N-15 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 NO3--N is diffused following NH4+-N and that separate diffusions should be performed for NH4+-N and (NH4+ + NO3-)-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 NO2- is completely removed from soil extracts by treatment with sulfamic acid before diffusion. A comparison of N-15 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.