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In Situ Estimation of the Gaseous Nitrous Oxide Diffusion Coefficient in a Sandy Loam Soil

Soils and Crops Research and Development Center, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC G1V 2J3, Canada

Jean Caron

Département des sols et de génie agroalimentaire, Université Laval, Sainte-Foy, QC G1K 7P4, Canada

*Corresponding author(vanbochovee{at}em.agr.ca).

ABSTRACT

The estimation of gas exchanges with the soil is important when assessing gas emission to the atmospheric environment or gas consumption by the soil. This estimation often requires the measurement of the diffusion coefficient of the investigated gas. The aim of this study was to develop an experimental method creating limited soil disturbance and based on simple calculation procedures to determine the diffusion coefficient, D_s, for N₂O in soils. This objective was achieved using a N₂O mass flow meter that supplied a continuous and constant flow of gas from below the soil surface until a steady-state N₂O concentration gradient with depth was obtained. The coefficients of diffusion were calculated daily for 6 d, under steady-state concentration gradient, using Fick's first law. The mean value of the diffusion coefficient for N₂O was 1.26 x 10^-6 m² s^-1 (CV = 25%) at 0.16 m³ m^-3 of air-filled porosity. This value was validated by a transient-state laboratory method using soil cores and compared with empirical estimations of D_s from air-filled porosity determinations. The two experimental methods produced similar values for D_s and both provided more accurate estimates than the empirical models. The proposed method can be applied to other gases provided an empirical model relating D_s to D_o (gas diffusivity in free air) is first established. A theoretical study identified the threshold air-filled porosity values below which the flux density must be adjusted to maintain the validity of the D_s calculations.

Received for publication June 1, 1997.

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