The elusiveness of measuring nitrite in soils: fast chemical reactions or inadequate extraction methods?

Soil nitrite (NO2-) is an important source of nitrous acid to the atmosphere as well an intermediate in the nitrification and denitrification pathway. Few studies, however, have directly linked NO2- pools with gaseous N emissions, presumably because NO2- is too reactive and seldom present in detectable amounts in soils. Here, we question whether the elusiveness of soil NO2- is due to its fast reactivity or to methodological problems associated with conventional 2M KCl extraction techniques. In our approach, we extracted soils along a pH gradient (5-8) in both deionized water (DIW) and 2M KCl. We used spikes to evaluate recovery efficiency of NO2- and used conventional colorimetric methods for the determination of NO2- in KCl as well as ion chromatography for DIW extracts. We found that 2M KCl underestimated soil NO2- pools by as much as two orders of magnitude when compared to DIW, but that these differences decreased as soil pH increased. In acidic soils, 2M KCl favored the transformation of NO2- to nitric oxide. Because 2M KCl lowers the pH of soil extractions by one unit when compared to DIW, this tenfold increase in acidity appears to favor transformations of NO2- into gaseous N products. DIW was a better alternative for the determination of soil NO2- as evidenced by the efficient recovery of added NO2- spikes in addition to native soil NO2- pools. However, filtration (0.45 μm) must be used to remove interferences caused by colloidal suspensions in DIW. Determination of NO2- pools in DIW may help establish direct relationships between seasonal changes in NO2- pools and gaseous N emissions from soils.