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Simultaneous Transport of Nitrate and Gaseous Denitrification Products in Soil¹

ABSTRACT

A pulse of NO₃^- fertilizer tagged with ¹⁵N was applied to 100 cm long Yolo loam (Typic Xerorthents) topsoil and subsoil columns maintained uniformly unsaturated at soil-water pressure heads between –20 and –140 cm of water at soil-water fluxes between 1.0 and 0.1 cm day^-1. Nitrate, molecular nitrogen, and nitrous oxide from the applied fertilizer were measured as a function of soil depth and time until the NO₃^- pulse eluted from the column. An analytical solution describing the transport and transformation of NO₃^- was used to determine values for the first-order denitrification rate constants within the columns. A numerical solution of the coupled equations for transport and transformation of NO₃^- and diffusion of the gaseous denitrification products was compared with measured N₂ and N₂O concentration profiles within columns. The gaseous concentration profiles were very much dependent upon values of the denitrification rate constant and the soil gaseous diffusion coefficient. Values of the soil gaseous diffusion coefficient, more than an order of magnitude smaller than those values measured in the upper part of the column, were required to approximately fit the numerical solution to the measured gas concentration profiles.

¹ Contribution from the Dep. of Land, Air, and Water Resources, Univ. of California, Davis.

² Assistant Professor of Soil Science and Associate Professor of Water Science and Civil Engineering, respectively, Dep. of Land, Air, and Water Resources, Univ. of California, Davis, CA 95616

Received for publication April 22, 1976. Accepted for publication July 7, 1976.