Forced Diffusion soil flux: A new technique for continuous monitoring of soil gas efflux

Measurements of soil carbon dioxide efflux provide critical information on soil carbon balance. In light of increasing interest in monitoring carbon balance of northern soils, it is important that we develop new methodologies that are better suited to long-term, remote, and off-grid deployments. In this study, we describe a Forced Diffusion (FD) dynamic chamber, in which a gas permeable membrane passively regulates mixing of atmosphere and soil air in the chamber, in place of the active pumping system inside a regular dynamic efflux chamber system. We use a combination of methodologies to explore the FD chamber technique, including numerical modelling, laboratory benchmarking, mesocosm testing, and year-round field studies in northern temperate ecosystems. Not surprisingly, FD chambers are functionally similar to regular dynamic chambers, but the passive regulation of gas flow means that internal concentration sensors can be switched off between measurements, thereby achieving very low power consumption and high reliability. In numerical modelling experiments and controlled lab tests, FD chambers deliver data that is comparable to commercially available instruments. As with other efflux techniques, calibration, design geometry, and deployment methodology are critical issues for generating good accuracy and precision with FD chambers.