Impact of sample preservation methods on the extraction of inorganic nitrogen by potassium chloride

Accurate measurement of soil mineral nitrogen (N) is essential for making precise N recommendations for grain maize (Zea mays L.) production. Analysis of soil inorganic N is complicated by the fact that these forms of N change rapidly during the sample-processing period. Studies were conducted to evaluate the effects of soil-preservation methods on the changes in inorganic N concentration of soil samples. In 1999 and 2000, soil samples at 0 to 20 cm depth were collected from eight locations representing different soil types. Six preservation methods were evaluated, including an immediate extraction in the laboratory for NH4-N and NO3-N determination, and extraction of samples stored under different preservation methods: frozen at -15 degrees C for 2 months, air dried in a greenhouse for 24 h, oven dried at 20 degrees C for 24 h, oven dried at 40 degrees C for 24 h, and air dried at room temperature at 22 degrees C for 24 h. All preservation methods caused a significant increase in NH4-N and, to a smaller degree, NO3-N concentrations, except freezing, which did not increase NH4-N in 1999. Compared with analysis of fresh samples, soil total inorganic N (NH4-N + NO3-N) was increased, on average, by 10.0 microgram g(-1) in 1999 and 1.4 microgram g(-1) in 2000 for the frozen treatment. Air-drying at room temperature produced the smallest increase, 4.0 microgram g(-1) in 1999 and 2.4 microgram g(-1) in 2000, followed by oven drying at 20 degrees C (4.6 microgram g(-1) in 1999 and 3.9 microgram g(-1) in 2000) and oven drying at 40 degrees C (4.3 microgram g(-1) in 1999 and 7.1 microgram g(-1) in 2000). Air-drying in the greenhouse produced the greatest increase, 6.0 microgram g(-1) in 1999 and 6.5 microgram g(-1) in 2000. Results of this study indicate that air-drying at room temperature (22 degrees C) for 24 h with subsequent storage in sealed polyethylene containers is a reasonable and relatively reliable method in preserving soil samples for inorganic N, especially for NO3-N in low-mineral N-content soils.