Respiration can be measured either as an increase in CO₂ or as a decrease in O₂. Because CO₂ can be accurately measured by infrared absorption, CO₂ has been widely used for measurements of soil respiration. However, CO₂ is 30 times more soluble in water than O₂ so the sensitivity of gas phase CO₂ to temperature is 30 times greater than for O₂. This means that measured fluxes of CO₂ from soils can be the result of non-metabolic changes in CO₂ in the gas phase of soil. These considerations are particularly significant when using the gradient flux method of measuring soil respiration because continuous measurement of either CO₂ or O₂ in soil gas is required. We calculated the effects of temperature, barometric pressure, pH, and humidity on CO₂ and O₂ concentrations in soil gas. The effect of temperature on gaseous CO₂ is complex at higher pH values because CO₂ gas is also in equilibrium with bicarbonate in the soil solution.

Results/Conclusions

At 25 °C and 0.1 % CO₂ (1000 ppm), our calculations indicate that CO₂ should increase by 0.0015 % CO₂ per °C (15 ppm per °C), and the effect of temperature on O₂ is negligible. Measurements in autoclaved sand (using an infrared sensor for CO₂ and galvanic-cell O₂ sensors that can resolve 0.001 % O₂ (10 ppm)) confirmed these results. The increased sensitivity of infra-red measurement of CO₂ is largely offset by the increased temperature correction for non-metabolic effects of CO₂. Measurement of O₂ should be used to supplement CO₂-based methods of soil respiration.