The gradient method has been applied to many physical measurements and is becoming a more commonly used approach to determine soil gas (e.g., C02) efflux. Gas diffusion coefficient (DP) and gas concentration gradient are the two key parameters in Fick' Law for applying the gradient efflux approach. Field observed data show good agreement between gradient and chamber methods days after water application. The assumptions of a linear concentration gradient and uniform soil water content profile may be violated however, by non-uniform wetting of the soil, which in turn may lead to non-linear gas concentration gradients in the soil. These non-ideal conditions are typically associated with short-term responses after rainfall or irrigation events. 0ther perturbations could include application of nutrients that increase localized respiration activity. Appropriate description of the water-content dependent diffusion coefficient, DP, requires some knowledge of available models and limitations on their use. Here a power function model worked well for describing both our field and laboratory experimental data. Comparison of surface chamber efflux with gradient-based measurements indicated estimates from concentrations down to 15 cm were appropriate for flux determination using the gradient method when 10 mm water was applied. Appropriate application of the C02 gradient-based method should consider the impact of recent water application on the C02 concentration distribution in soil profile as well as the possibility of a wetter soil layer reducing overall flux. These two aspects limit application of the gradient method for soil C02 efflux estimation.
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