Direct-diffusion methods for inorganic-nitrogen analysis of soil

Determinations of exchangeable NH₄, NO₃, and NO₂ in soil normally require the preparation of an extract, which is usually obtained with a neutral salt solution such as 2 M KCl. In cases where analytical speed and convenience are key factors, as in routine soil testing, a more practical approach would be to carry out the determination directly on the soil sample itself. Studies were conducted to ascertain whether this approach could be employed using Mason-jar diffusion methods previously developed for inorganic-N analysis of soil extracts. Direct diffusions were performed with MgO to recover NH₄-N, or with MgO and Devarda's alloy to recover (NH₄ + NO₃ + NO₂)-N, from 1- to 5-g samples of a wide variety of air-dried soils, following treatment of the soil with sufficient 2 M KCl to give a 1:10 ratio of soil to solution. Up to 4 mg of inorganic N was recovered quantitatively in 18 h to 6 d at room temperature, or in 1.75 to 5.5 h with gentle heating on a hot plate (45-50°C). Analyses by direct diffusion were in close agreement (r = 0.993-0.999) with results obtained by diffusion of soil extracts, and recoveries ranged from 97 to 102% when direct diffusions were performed on samples that had been treated with NH₄-N or NO₃-N. Besides being quantitative, direct diffusions were found to have potential for N-isotope analysis. In diffusing ¹⁵N-treated samples, isotope-ratio analyses of NH₄-N or (NH₄ + NO₃ + NO₂) - N were accurate to within 3%. The latter finding virtually precludes interference by labile organic N or nonexchangeable NH₄N; however, interference did lead to an increase in inorganic-N analyses when diffusion was prolonged beyond the recommended period.