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a Dep. of Natural Resources and Environ. Sci., Urbana, IL USA
b Dep. of Crop Sci., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801 USA
mulvaney{at}uiuc.edu
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
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 NH4–N; however, interference did lead to an increase in inorganic-N analyses when diffusion was prolonged beyond the recommended period.
Abbreviations: ARA–MS, automated Rittenberg apparatus–mass spectrometry • CEC, cation-exchange capacity • CV, coefficient of variation • SD, standard deviation
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