CO2 fluxes in subtropical dryland soilsa comparison of the gradient and the closed-chamber method

On the plot scale, closed dynamic chamber methods (CDCM) are the most popular technique to directly measure CO2 fluxes from the soil surface to the atmosphere. However, the gradient method (GM) provides several advantages, which refer to the underground processes contributing to the CO2 production that cannot be investigated by the CDCM. To evaluate the suitability of the GM in dryland soils, we compared feasibility and quality of results for both methods. The GM is based on Fick's law and requires knowledge on diffusion properties of the soil, concentration gradients between soil and atmosphere, and the air-filled porosity. Our study was conducted on two sites along the Okavango River, one in Namibia (semi-arid) and the other in Angola (semi-humid) with comparable sandy soil texture. The CO2 concentration profile was determined by collecting soil gas samples from different soil depths. CO2 efflux was measured with a vented steady-state closed chamber system. Soil gas diffusivities were measured in lab experiments using diffusion chambers and undisturbed soil cores. Modeled diffusivities were predicted according to six popular models based on air-filled porosity and total porosity as input parameters. Results show strong agreement between CDCM and GM fluxes based on measured diffusivities. However, with modeled diffusivities overestimation of fluxes for most of the tested models, especially at high air-filled porosity, were detected. We conclude that the GM offers a valuable tool for flux estimates on the pedon scale in dry ecosystems particularly in combination with measured diffusivities and includes the possibility for investigating subsurface processes involved in the CO2 production.