Wind-induced error in the measurement of soil respiration using closed dynamic chambers

We assess errors in soil respiration fluxes of CO2 obtained using the closed dynamic chamber method. Particular attention is given to small pressure gradients between the chamber headspace and the external environment that may induce mass flow of soil air, leading to overestimation of soil respiration. These pressure gradients develop as air movement creates a Venturi effect at the vent that is designed to insure pressure equilibration, leading to aspiration of air from within the chamber. During field experiments at the Harvard Forest, the Venturi effect produced pressure gradients of approximately 1 Pa per 1 m s−1 for a chamber sealed to an impermeable plate, but no pressure gradient was observed in an identical system deployed on the forest soil. Mass flow of soil air compensated for the wind-driven pressure gradients, and increases in CO2 fluxes exceeding a factor of 2 were observed in response to wind events even under a dense forest canopy. The high porosity of forest soils allows pressure artifacts induced by winds or by sampling flows to perturb the diffusive flux from soils, potentially affecting virtually all chamber methods. Associated errors in soil respiration measurements must be addressed through chamber design and evaluation.