The effects of chamber pressurization on soil-surface CO2 flux and the implications for NEE measurements under elevated CO2

Soil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas-exchange experiments and a soil CO2 transport model to characterize the effects of c hamber pressure on soil CO2 efflux in an annual California grassland. For g reater than ambient chamber pressures, experimental data show that soil-sur face CO2 flux decreases as a nonlinear function of increasing chamber press ure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO2 efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO2 tran sport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO2 from the soil by generating a downward now of air t hrough the soil air-filled pore space. This advective flow of air reduces t he CO2 concentration gradient across the soil-atmosphere interface, resulti ng in a smaller diffusive flux into the chamber head space. Simulations als o show that the reduction in diffusive flux is a function of chamber pressu re, soil moisture, soil texture, the depth distribution of soil CO2 generat ion, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portio n of the frequently observed increase in net ecosystem carbon uptake under elevated CO2 may be an artifact resulting from the impact of chamber pressu rization on soil CO2 efflux.