Closed chamber systems underestimate soil CO ₂ efflux

Summary Chamber systems are widely used to measure soil CO 2 efflux, using either a closed or an open gas exchange methodology. In comparisons between these two methods fluxes measured with closed systems are often found to be around 10% less than those measured with open systems. One reason why closed chambers might systematically underestimate the true efflux rate is that the effective chamber volume is larger than the volume of the chamber alone, i.e. it includes the volume of air‐filled spaces in the soil. In tests carried out in a northern forest, this error resulted in a closed chamber underestimating the true efflux by up to 30%, with an average underestimation of around 10%.