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Improving nitrite analysis in soils: Drawbacks of the conventional 2 M KCl extraction

  • Author(s): Homyak, PM
  • Vasquez, KT
  • Sickman, JO
  • Parker, DR
  • Schimel, JP
  • et al.
Abstract

Soil nitrite (NO ) is an important source of nitrous acid to the atmosphere as well an intermediate in nitrification and denitrification. Few studies, however, have directly linked NO2- pools with N emissions because NO is reactive and seldom detectable in soils. Here, we test whether the elusiveness of soil NO is due to its reactivity or to problems associated with conventional 2 M KCl extractions. We extracted acidic, neutral, and alkaline soils (pH 5.4- 8.2) in 2 M KCl, pH-8-adjusted 2 M KCl, and deionized water (DIW). Unbuffered KCl consistently underestimated soil NO compared with DIW; soils with lower pH had lower NO in unbuffered KCl than in DIW water. In acidic soils, unbuffered KCl favored the transformation of NO to nitric oxide. Because KCl lowers the pH of extracts by ~1 unit, this increase in acidity likely favored the transformation of NO to gaseous N products. Although buffered KCl minimizes NO destruction, it can cause colorimetric interferences when done on small soil samples (4 g). Deionized water extractions offer an alternative for measuring NO in small samples, but filtering beyond the traditionally used 2.5-mm filter paper is necessary to remove suspended solids. Despite its widespread use, unbuffered 2 M KCl should not be used for the analysis of soil NO . 2 2 2 2 2 2 2 2 2 2 - - - - - - - - - -

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