Nitrite Transformations during Soil Extraction with Potassium Chloride
Stevens, R. J. | Laughlin, Ronnie J.
Nitrite is a key intermediate in both NH⁺₄ oxidation and NO⁻₃ reduction. There is increasing concern over its origin and significance in food and water. Measuring NO⁻₂ concentrations in soils requires extraction procedures, KCl (2 M) being the most commonly used extractant. We investigated the effect of soil/extractant ratio, extraction procedure, shaking time, and pH on the transformations of ¹⁵NO⁻₂ from a range of grassland soils. We measured the recovery of NO⁻₂ and ¹⁵N as ¹⁵NO⁻₂ or ¹⁵NO⁻₃. Nitrite destruction and pool dilution occurred at 1:10 and 1:1 soil/extractant ratios. Using a 1:1 soil/extractant ratio, average ¹⁵NO⁻₂ recovery for six soils was 86% with a blending procedure (10 min), and 32% with a shaking procedure (70 min). About two-thirds of the ¹⁵NO⁻₂ lost during extraction appeared as ¹⁵NO⁻₃. When the pH of the soil suspension was adjusted with KOH to 7.8, recovery of NO⁻₂ was not significantly less than 100% for reaction times up to 40 min. Pool dilution and decomposition of NO⁻₂ appeared to be due to chemical rather than microbial reactions. When 20 soils were tested using the blending procedure and extractant pH of 8.0, 14 gave recoveries >97% and an additional six gave recoveries >90%. One of the mineral soils giving >97% recovery was extracted at two pH values, and NO⁻₂ stability measured in filtered extracts during 28 d of storage at 4°C. Nitrite was more stable in extracts at pH 5.6 than 8.0. To ensure at least 95% recovery of ¹⁵NO⁻₂, extracts at pH 8.0 had to be filtered through a 0.7-µm filter and analyzed within 2 d of extraction.
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