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Improving Nitrite Analysis in Soils: Drawbacks of the Conventional 2 M KCl Extraction


Soil nitrite (NO2-) 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 NO2- is reactive and seldom detectable in soils. Here, we test whether the elusiveness of soil NO2- 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 NO2- compared with DIW; soils with lower pH had lower NO2- in unbuffered KCl than in DIW water. In acidic soils, unbuffered KCl favored the transformation of NO2- to nitric oxide. Because KCl lowers the pH of extracts by ~1 unit, this increase in acidity likely favored the transformation of NO2- to gaseous N products. Although buffered KCl minimizes NO2- destruction, it can cause colorimetric interferences when done on small soil samples (4 g). Deionized water extractions offer an alternative for measuring NO2- 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 NO2-.

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