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Measurement of Soil Gaseous Diffusion Coefficients by a Transient-State Method with a Time-dependent Surface Condition¹

ABSTRACT

Precise measurement of the gaseous diffusion coefficient in soils is necessary for calculating the flux of gas resulting from soil respiration, denitrification, or fumigation. Gaseous diffusion coefficients of soil columns and a field soil were determined by fitting a solution of the transient-state diffusion equation for a time-dependent surface condition to measured N₂ concentration profiles. The time-dependent surface condition was experimentally imposed by pumping argon at a constant rate into a chamber placed within the soil and on the soil surface. The argon and air mixture exited the chamber through several exit ports. The N₂ concentration at the soil surface and within the soil profile was determined several times within a 5-hour period after initiation of the argon flow. A solution of the diffusion equation for a non-constant initial condition can also be used for determining the diffusion coefficient for cases where the initial gas concentration is not constant with depth. Diffusion coefficients for a field soil determined from the transient-state solutions compared favorably with those determined from laboratory measurements on undisturbed soil cores and field measurements of CO₂ flux.

¹ Contribution from the Dep. of Land, Air and Water Resources, Univ. of California, Davis, CA 95616.

² Assistant Professor of Soil Science and Research Assistant, respectively. Dep. of Land, Air and Water Resources, University of California, Davis, CA 95616.

Received for publication August 9, 1976. Accepted for publication November 8, 1976.