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Nitrification and Denitrification during Miscible Displacement in Unsaturated Soil¹

ABSTRACT

Nitrification and denitrification during the leaching of a pulse of NH₄NO₃ applied to the surface of soil columns are analyzed experimentally and theoretically. Experimentally, unsaturated soil columns of two lengths were leached steadily with a tracee solution containing NH₄NO₃ while the air-filled portions of the columns were being flushed simultaneously with air. During these steady state conditions, a pulse of tracer solution containing either ¹⁵NH₄⁺ or ¹⁵NO₃^- was leached through the columns. The words tracee and tracer signify that both solutions were chemically identical with the latter containing a greater proportion of ¹⁵N. Steady-state tracee concentration distributions within the columns as well as the transient tracer concentration distributions in the effluent were analyzed assuming first-order kinetics. Values of both the rate coefficients and the concentration distributions stemming from the tracee and tracer measurements were in general agreement. The agreement between the measured and theoretical results are discussed in terms of soil water properties and the kinetics of microbial populations.

¹ Contribution from the Department of Water Science and Engineering, Department of Soils and Plant Nutrition, Univ. of California, Davis 95616.

² Postgraduate Water Scientist and Soil Scientist, Univ. of California, Davis; Soil Physicist, Research Wing, Univ. of Agriculture and Technology, Orissa, India; and Professors of Water Science, Univ. of Calif., Davis, respectively.

Received for publication February 15, 1974. Accepted for publication June 6, 1974.