CO2 flux measurement using soda lime: correction for water formed during CO2 adsorption

Ecology, June, 1998 by Paul Grogan

Carbon dioxide release from soil is an indicator of microbial and root activity and as such is an essential component of terrestrial carbon budgets and models of ecosystem carbon cycling. The soda-lime technique has been used extensively for [greater than]30 yr to measure C[O.sub.2] fluxes from soil under field conditions (e.g., Monteith et al. 1964). Inaccuracies with the method arise because the C[O.sub.2] adsorption rate of soda lime is rarely in equilibrium with the efflux rates being measured. The use of a calibration curve (see below) is necessary to compensate for this error. Despite this limitation, soda lime has distinct advantages for making field estimates of C[O.sub.2] fluxes: (1) it can readily provide single integrated measures over a daily time scale and thus incorporate the effects of diurnal fluctuations in abiotic variables that control C[O.sub.2] release; (2) it is robust and economical, making it more appropriate for the large numbers of replicate field measurements necessary to account for the enormous spatial heterogeneity associated with soil surface C[O.sub.2] effluxes (Cropper et al. 1985, Raich et al. 1990, Rochette et al. 1991). For these reasons, the soda-lime technique is likely to continue to have applications for in situ field measurements of soil respiration. Protocols for its use are described in most major soils methods texts (e.g., Soil Science Society of America: Anderson 1982, Zibilske 1994). Furthermore, the technique has been used to report soil respiration fluxes in at least 12 studies in major ecological journals since 1988.

Soda lime is a variable mixture of sodium and calcium hydroxides that reacts with C[O.sub.2] to form carbonates. The amount of C[O.sub.2] adsorbed by soda lime exposed beneath an inverted chamber placed over the soil surface is proportional to the increase in soda-lime dry mass during the sampling period. In recent years, many researchers have preferred to use a "dynamic" method in which the rate of change in C[O.sub.2] concentration of air circulated through an inverted chamber is measured using an infrared gas analyzer (IRGA). Laboratory studies with known fluxes and simulated soil surfaces (e.g., Nay et al. 1994) have demonstrated that better accuracy is achieved with the dynamic approach because the soda-lime adsorption rate is rarely at equilibrium with the flux being measured. At flux rates reported as [less than or equal to]1.6 g C[O.sub.2]-C[multiplied by][m.sup.-2][multiplied by][d.sup.-1], Nay et al. demonstrated that soda lime overestimated fluxes. Under such conditions, adsorption of C[O.sub.2] by soda lime lowered headspace concentrations below ambient thus enhancing the flux diffusion gradient. By contrast, underestimation occurred at a flux rate of 5.0 g C[O.sub.2][multiplied by]C[multiplied by][m.sup.-2][d.sup.-1]. At this value, C[O.sub.2] concentrations in the headspace greatly exceeded ambient levels, suggesting that soda-lime adsorption efficiency became progressively more limiting as flux rates increased. In the field, an extensive field comparison of soda lime and IRGA methods (Ewel et al. 1987) found that underestimation occurred at values [greater than] 1.4 g C[O.sub.2]-C[multiplied by][m.sup.-2][multiplied by][d.sup.-1] [ILLUSTRATION FOR FIGURE 1 OMITTED]. Almost identical logarithmic regression equations relating results from the two techniques have now been derived independently (Ewel et al. 1987, Haynes and Gower 1995) that allow correction for underestimation by soda lime at high fluxes. The following discussion outlines an inherent error in the method used to calculate soda-lime fluxes that modifies the slope of the soda lime/IRGA relationship.

Edwards (1982) conducted a review of the soda-lime method in which guidelines for determining the appropriate exposed surface area and quantity of soda lime were described. Soda-lime adsorption efficiency was tested over extended exposures and shown to decline once the initial mass had increased by 7%. Attention was also drawn to the fact that a correction factor is required to account for water that is formed when soda lime reacts with C[O.sub.2]:

2 NaOH C[O.sub.2] [right arrow] [Na.sub.2]C[O.sub.3] [H.sub.2]O

Ca[(OH).sub.2] C[O.sub.2] [right arrow] CaC[O.sub.3] [H.sub.2]O.

For every mole of C[O.sub.2] that is chemically bound to soda lime, a mole of water is formed that is subsequently evaporated during drying. Thus, the increase in dry mass measured before and after exposure underestimates C[O.sub.2] absorbed by a factor of 18/44 or 40.9%. Edwards (1982) proposed that a correction factor of 1.41 should be applied to the measured mass difference in order to calculate the actual mass of C[O.sub.2] absorbed. In fact, Tesarova and Gloser (1976) had previously reached the same conclusion. However, this conclusion is in error because underestimation by 40.9% implies that the measured mass difference is 59.1% (100 - 40.9) of the true mass of C[O.sub.2] absorbed. The appropriate correction factor to be applied to the measured mass difference in order to calculate the true mass of C[O.sub.2] absorbed is 1.69 (i.e., 100/59.1).