1. A new open-top chamber for measuring CO2 efflux from the soil is reported here. The new design enables measurement of the equilibrium CO2 efflux, when there is no detectable pressure difference between the chamber and outside nor leakage of CO2 into or out of the chamber.
2. In previous dynamic-chamber techniques, the measured CO2 efflux is dependent on the pressure difference between the inside and outside of the chamber, and a negative pressure difference of –1Pa may cause an order of magnitude increase in measured CO2 efflux. Although the measured CO2 efflux is less sensitive to a positive pressure difference than to a negative one, a positive pressure difference of even a few tenths of a Pa will lead to a considerable underestimation in soil CO2 evolution.
3. The influence of pressure difference on measured CO2 efflux is negligible in the new design and the estimated CO2 efflux is close to the undisturbed soil respiration rate. Flow rates up to 8lmin–1, or air movement over the soil surface up to 55cmmin–1, will not affect CO2 evolution from the soil. The influence of pressure difference is related to the type of soil being measured and this has also been reported here for the new design.