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Differentiation of Soil Nitrogen Fractions Using a Kinetic Approach¹

ABSTRACT

The chemically induced release of NH₄-N from four Delaware soils was monitored over time. The extractants used were 0.02 M KMnO₄-0.5 M H₂SO₄ at 297 K and 0.01 M CaCl₂ at 368 K. Data were analyzed using first-order kinetics. The kinetic approach presented does allow direct differentiation of two NH⁺₄ releasing reactions. Two simultaneous first-order reactions were needed to describe three of the soils, while a single equation described the remaining soil. The equations were considered to represent the release of NH⁺₄ from an inorganic and an organic source. Using this approach, estimates of these pools were made and compared with estimates based on traditional batch techniques. Extraction with acid-KMnO₄ yielded kinetic estimates of the inorganic and organic pools, which exceeded the batch estimates of these pools. Kinetic estimates based on extraction with CaCl₂ were similar to batch estimates of inorganic N, but much less than either acid-KMnO₄ or batch estimates of organic N. Neither extractant gives direct evidence for the existence of a chemically active organic pool; however, there is indirect evidence for such a pool.

¹ Published as Miscellaneous Paper no. 1153 of the Delaware Agric. Exp. Stn. Contribution no. 210 of the Dep. of Plant Science, Univ. of Delaware, Newark, DE 19717-1303.

² Former University Graduate Research Fellow and Associate Professor of Soil Chemistry, respectively. The senior author is now Research Soil Chemist at E.I. Du Pont De Nemours, Wilmington, DE 19898.