NITRITE TRANSFORMATIONS DURING SOIL EXTRACTION WITH POTASSIUM-CHLORIDE

Nitrite is a key intermediate in both NH4+ oxidation and NO3- reductio n. There is increasing concern over its origin and significance in foo d and water. Measuring NO2- concentrations in soils requires extractio n procedures, KCI (2 M) being the most commonly used extractant. We in vestigated the effect of soil/extractant ratio, extraction procedure, shaking time, and pH on the transformations of (NO2-)-N-15 from a rang e of grassland soils. We measured the recovery of NO2- and N-15 as (NO 2-)-N-15 or (NO3-)-N-15. Nitrite destruction and pool dilution occurre d at 1:10 and 1:1 soil/extractant ratios. Using a 1:1 soil/extractant ratio, average (NO2-)-N-15 recovery for six soils was 86% with a blend ing procedure (10 min), and 32% with a shaking procedure (70 min). Abo ut two-thirds of the (NO2-)-N-15 lost during extraction appeared as (N O3-)-N-15. When the pH of the soil suspension was adjusted with KOH to 7.8, recovery of NO2- was not significantly less than 100% for reacti on times up to 40 min. Pool dilution and decomposition of NO2- appeare d 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 valu es, and NO2- stability measured in filtered extracts during 28 d of st orage at 4 degrees C. Nitrite was more stable in extracts at pH 5.6 th an 8.0. To ensure at least 95% recovery of (NO2-)-N-15, extracts at pH 8.0 had to be filtered through a 0.7-mu m filter and analyzed within 2 d of extraction.