Reduction of bias in static closed chamber measurement of δ¹³C in soil CO₂ efflux

Abstract

The (13)C/(12)C ratio of soil CO(2) efflux (delta(e)) is an important parameter in studies of ecosystem C dynamics, where the accuracy of estimated C flux rates depends on the measurement uncertainty of delta(e). The static closed chamber method is frequently used in the determination of delta(e), where the soil CO(2) efflux is accumulated in the headspace of a chamber placed on top of the soil surface. However, it has recently been shown that the estimate of delta(e) obtained by using this method could be significantly biased, which potentially diminish the usefulness of delta(e) for field applications. Here, analytical and numerical models were used to express the bias in delta(e) as mathematical functions of three system parameters: chamber height (H), chamber radius (R(c)), and soil air-filled porosity (theta). These expressions allow optimization of chamber size to yield a bias, which is at a level suitable for each particular application of the method. The numerical model was further used to quantify the effects on the delta(e) bias from (i) various designs for sealing of the chamber to ground, and (ii) inclusion of the commonly used purging step for reduction of the initial headspace CO(2) concentration. The present modeling work provided insights into the effects on the delta(e) bias from retardation and partial chamber bypass of the soil CO(2) efflux. The results presented here supported the continued use of the static closed chamber method for the determination of delta(e), with improved control of the bias component of its measurement uncertainty.

Effects of soil type and composition of rhizodeposits on rhizosphere priming phenomena

“…The values measured in the control soils in Experiment B were significantly less negative than those in the control soils in Experiment A. Following Nickerson and Risk (2009) and Ohlsson (2010), we suppose this is due to isotopic fractionation in transient-state diffusion of respired CO 2 through the soil to the purged microcosm chambers, which is expected to be greater in Experiment B for reasons discussed in Appendix A. In calculations given in Appendix A, we show that the apparent error in δ 13 C values will cause the size of the priming effect to be under-estimated, but the trends across the treatments and over time are unchanged.…”

“…8 ‰ more positive than those in Experiment A. We suppose this is due to isotopic fractionation in the transient-state diffusion of respired CO 2 through the soil to the microcosm chambers, as discussed by Nickerson andRisk (2009) andOhlsson (2010). In systems using purged chambers, such as ours, this can produce a positive δ 13 C bias of up to 15 ‰ (Nickerson and Risk, 2009).…”

Challenges in measuring the δ13C of the soil surface CO2 efflux

“…However, in dry soils air movement through the surface of the profile may still be an issue and the skirt alters the effective footprint of the chamber. In more recent work, Ohlsson [67] mathematically evaluated a number of soilchamber sealing designs, including a chamber skirt as above, single walled chambers inserted to 0.002 and 0.025 m into the soil surface and a double walled chamber. They concluded that the latter was the most effective, but this approach has yet to be tested in the field; more work in this area is clearly desirable.…”

“…These feedbacks arise from non-steady-state conditions, with the build up of CO 2 in the chamber influencing the diffusion of soil CO 2 and its isotopologues, which would be an issue for all chamber systems, to a greater or lesser extent, [66] with dynamic chambers (see later) offering possibly the least disturbance. Ohlsson [67] has recently argued that, despite these findings, by careful selection of the chamber size in terms of height and area covered, the bias introduced by a closed chamber can be minimised to a point where useful isotope data can still be obtained; a modelling approach was used here, building on the work of Livingston et al [43] and Nickerson and Risk.…”

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

“…Furthermore, Kayler et al (2010b) demonstrated in a field study the interrelation between perturbations of CO 2 in soil pores and aboveground measurement techniques for δ 13 C of soil respiration. Numerical approaches considering diffusion of CO 2 in soil air have been applied to simulate the impact of transient changes in environmental variables (Nickerson and Risk, 2009a;Moyes et al, 2010) or the deployment of respiration chambers (Nickerson and Risk, 2009b,c;Ohlsson, 2010) on δ 13 C efflux and, again, the disequilibrium effect. For example, CO 2 accumulating in the headspace of closed chambers and associated chamber-soil feedbacks can cause deviation of Keeling plots (Keeling, 1958) from linearity (Nickerson and Risk, 2009b;Kammer et al, 2011).…”