A Numerical Examination of ¹⁴CO₂ Chamber Methodologies for Sampling at the Soil Surface

Abstract

Radiocarbon is an exceptionally useful tool for studying soil-respired CO 2 , providing information about soil carbon turnover rates, depths of production, and the biological sources of production through partitioning. Unfortunately, little work has been done to thoroughly investigate the possibility of inherent biases present in current measurement techniques, like those present in δ 13 CO 2 methodologies, caused by disturbances to the soil's natural diffusive regime. This study investigates the degree of bias present in four 14 C sampling chamber methods using a three-dimensional numerical soil-atmosphere CO 2 diffusion model. The four chambers were tested in an idealized, surrogate reality by assessing measurement bias with varying Δ 14 C and δ 13 C signatures of production, collar lengths, soil biological productivity rates, and soil diffusivities. The static and Iso-FD chambers showed almost no isotopic measurement bias, significantly outperforming dynamic chambers, which demonstrated biases up to 200‰ in some modeled scenarios. The study also showed that 13 C and 14 C diffusive fractionation are not a constant multiple of one another, but that the δ 13 C correction still works in diffusive scenarios because the change in fractionation is not large enough to impact measured Δ 14 C values during chamber equilibration.

An assessment of chamber 14C methodologies for sampling aquatic CO2 evasion

“…This suggests that different collection methodologies did not result in different 14 C ages. Furthermore, it supports our hypothesis that any isotopic fractionation effects caused by the different chamber sampling approaches are either too small to be significant (as reported by Egan et al, ), or are corrected for by using the existing 13 C normalization approach (Stuiver & Polach, ).…”

“…Because the conditions in the floating chamber differ from the ambient atmosphere, it has been argued that the isotopic composition of the CO 2 lost from the water surface will be altered (see Billett & Garnett, 2010). For example, the mass differences of individual carbon isotopes are known to affect rates of diffusion of CO 2 in air, with 12 CO 2 diffusing at a rate of 1.044 and 1.088 times faster than 13 CO 2 and 14 CO 2 , respectively (Cerling, Solomon, Quade, & Bowman, 1991;Craig, 1953;Egan, Nickerson, Phillips, & Risk, 2014). Although such "mass-dependent" isotopic fractionation effects do alter the 14 C concentration relative to the other isotopes, conventional 14 C age results are corrected for this by normalizing to a standardized δ 13 C of −25‰ (Stuiver & Polach, 1977).…”

Isotopic fractionation corrections for the radiocarbon composition of CO<sub>2</sub> in the soil gas environment must include diffusion and mixing

“…Method 1 is actually acceptable for use in the case of surface flux chambers, because unlike soil CO 2 which will always differ from soil production soil-respired CO 2 , conservation of mass dictates that isotopic values of flux must represent soil production so long as the soil is in steady-state (Cerling et al, 1991). While radiocarbon surface flux data need no correction for transport 15 fractionation, researchers should be cautious when using surface flux chambers because they can cause isotopic disequilibrium (Albanito et al, 2012;Egan et al, 2014;Midwood and Millard, 2011;Nickerson and Risk, 2009a). As shown in the Egan et al (2014) study, static chamber methods (i.e.…”

“…Gaudinski et al, 2000;Schuur and Trumbore, 2006) This study suggested that the conventional Stuiver and Polach (1977) radiocarbon correction accommodated isotopic fractionation by the sieve. However, under non-steady state conditions, the conventional correction may not actually apply because the 14 CO 2 / 12 CO 2 fractionation factor will not always be a constant multiple of the 13 CO 2 / 12 CO 2 fractionation factor as 25 the system moves from one state to another (Egan et al, 2014).…”

“…Both the Torn and Southon (2001) study and ours highlight the importance of reassessing old isotopic approaches for new application environments. To date, only three known studies (Egan et al, 2014;Phillips et al, 2013;Wang et al, 1994) have accounted for 14 C diffusion-transport, though ours is the first to propose a straightforward and theoretically-robust correction 25 that replaces the Stuiver and Polach (1977) solution for the soil gas environment (Method 2).…”