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-μm 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−.