Improved Techniques for Long-term Continuous Measurement of Soil Respiration and Respiratory Quotient.

Aerobic respiration in plants and microbes 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, at the elevated CO₂ concentrations in soil, CO₂ is considerably more soluble in water than O₂ so the sensitivity of gas phase CO₂ to temperature is much greater than for O₂. This means that fluxes of CO₂ from soils can result from changing temperature even when respiration is negligible. 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 the carbonate system in soil solution. At 25 °C and 0.1 % CO₂ (1000 ppm), our calculations indicate that CO₂ should increase by 0.0015% per °C (15 ppm per °C).  The effect of temperature on O₂ is only 0.5 ppm per °C. Measurements in autoclaved sand (using an infrared sensor for CO₂ and galvanic-cell O₂ sensors that can resolve 10 ppm O₂) generally 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. When measured simultaneously, the ratio of the two gasses (the respiratory quotient) can be used to indicate the substrate being metabolized.