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Digestion of Plant Materials for the Determination of Total Nitrogen to Include Nitrate

Dep. of Plant Sciences, Univ. of Arizona, Tucson, AZ 85721
Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078-0507.

^* Corresponding author.

Many modifications in procedures for N determination by Kjeldahl digestion exist. However, few modifications include the quantitative recovery of NO^–₃-N, which is essential in ¹⁵N research. A digestion procedure is described in which reduced Fe and H₂SO₄ pretreatment is used for the quantitative recovery of NO^–₃-N. The procedure utilizes an aluminum block digester with a programmable control unit. The recommended program employes a stepwise temperature ramp up to 400°C and continues digestion 1 h after clearing for a total digestion time of 7.25 h. Two other digestion programs were compared with the recommended program. One program used a temperature ramp identical to the first, but digested 2 h after clearing for a total time of 8.25 h. A third digestion program used a shortened temperature ramp, attained a final temperature of 400°C, and was completed 1 h after clearing for a total time of 5.90 h. Wheat (Triticum aestivum L.) grain samples with up to 0.5 mg additional NH⁺₄-N and NO^–₃-N were used to evaluate these procedures. The recommended procedure provides complete recovery of grain N, and added N with superior levels of precision in comparison to the other digestion procedures evaluated. The digestion procedure using 5.90 h did not yield accurate analysis of grain N, indicating incomplete digestion. The 8.25-h digestion did not result in improvement of grain N results or added N recovery.

Received for publication June 19, 1987.