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Plant Physiology 69:32-36 (1982)
© 1982 American Society of Plant Biologists

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Articles

Minimizing Nitrate Reduction during Kjeldahl Digestion of Plant Tissue Extracts and Stem Exudates 1

APPLICATION TO 15N STUDIES

Gary M. Pace2, Charles T. MacKown3 and Richard J. Volk

Department of Soil Science, North Carolina State University, Raleigh, North Carolina, 27650

From 10 to 60% of the nitrate present in plant tissue extracts and stem exudates of corn (Zea mays L.) was found to be reduced during Kjeldahl digestion, even in the absence of added reducing agents. This reduction is of particular concern in [15N]nitrate assimilation studies, because it results in an overestimate of nitrate reduction. To overcome this problem, a method was developed for removing nitrate prior to Kjeldahl digestion, thereby preventing nitrate reduction. The procedure utilizes hydrogen peroxide for partial oxidation of organic matter in order to minimize the nitration of organic compounds. The free nitrates are then volatilized as nitric acid from concentrated sulfuric acid at 95°C. When the proposed method was used as a pretreatment to Kjeldahl digestion, less than 0.5% of the applied nitrate was recovered in the reduced nitrogen fraction of plant tissue extracts and stem exudates.

2 Present address: Allied Chemical. Syracuse Research Laboratory, P.O. Box 6, Solvay, NY 13209.

3 Present address: Department of Agronomy, United States Department of Agriculture, Science and Education Administration, Agricultural Research, University of Kentucky, Lexington, KY 40506.

1 Supported by Grant PCM-7703152 from the National Science Foundation. Paper No. 6804 of the Journal Series of the North Carolina Agricultural Research Service at Raleigh.






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