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Tuesday, 7 October 2008: 1:45 PM
George R. Brown Convention Center, 362DE
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.