Using the gradient method to determine soil gas flux: A review

• The gradient method yields information about the vertical flux profile of soil gas.
• The GM is suitable for CO2 production and CH4 consumption in well-aerated soils.
• Determination of soil gas diffusivity is a major source of uncertainty.
• The flux calculation approach should be chosen carefully.
• Studies of N2O and CH4 production should be supported by chambers.

Gas exchange between soil and atmosphere represents a major component of global greenhouse gas fluxes. Chamber methods and micro-meteorological methods are well-established techniques to measure gas fluxes. The gradient method is not as widely used, but it has gained increased attention during the last decade. In this review we provide an overview of the gradient method, from the concept over different aspects of the application to the limitations and challenges of the method.Assuming gas diffusion as the dominant transport mechanism, gas flux in porous media such as soil or snow can be calculated based on the profiles of gas concentrations and soil gas diffusivity. A variety of systems has been used to determine the vertical gas profile depending on the objective of the respective study. The estimation of soil gas diffusivity is a major source of uncertainty. Soil gas diffusivity can be derived using diffusivity models, laboratory measurements or in situ approaches, e.g. the Radon method. Choosing a diffusivity model has to be considered carefully, since flux estimates are directly affected. Different approaches to calculate the gas flux have been introduced, from direct simple calculations to analytical and numerical solutions. Flux estimation is highly sensitive to the calculation procedure. It is important therefore to consider the implicit assumptions of each calculation approach.Several studies compared flux estimates measured by the gradient method and other methods. Good agreement was found in studies of CO2 production and methane consumption in soils, particularly in studies using near-continuous measurements of CO2. The relation was not as strong for fluxes of CH4 and N2O. Deviations were attributed to the possible coexistence of production and consumption of methane and N2O in the top soil layer.

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