Measuring the ¹³C content of soil‐respired CO₂ using a novel open chamber system

Abstract

Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the delta13C of soil-respired CO2. The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the delta13C of the chamber air every 120 s. CO2 of known delta13C value was passed through a column of sand and, using the chamber, the CO2 concentration stabilized rapidly, but 60 min was required before the delta13C value was stable and identical to the cylinder gas (-33.3 per thousand). Changing the chamber CO2 concentration between 200 and 900 micromol.mol(-1) did not affect the measured delta13C of the efflux. Measuring the delta13C of the CO2 efflux from soil cores in the laboratory gave a spread of +/-2 per thousand, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the delta13C of the surface efflux of up to 8 per thousand. The chamber was used to measure small transient changes (+/-2 per thousand) in the delta13C of soil-respired CO2 from a peaty podzol after gradual heating from 12 to 35 degrees C over 12 h. Finally, soil-respired CO2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed.

Challenges in measuring the δ13C of the soil surface CO2 efflux

“…However, we have tested this approach using sand columns and found that the measured d 13 C of the chamber air was enriched relative to that of the source gas. [46] Similarly, Kayler et al [65] have suggested from similar experiments that the d…”

“…We have also developed a dynamic-chamber system. In this case CO 2 -free air is used to maintain steady-state ambient [CO 2 ] levels within the chamber; this system is discussed later and fully described by Midwood et al [46] Measuring the isotopic content of the soil surface efflux,…”

“…We have recently described a dynamicchamber system to measure d 13 CR S . [46] Briefly, CO 2 -free air was supplied to a chamber from a cylinder of compressed air via self-indicating soda lime (to remove the CO 2 ), using a mass flow controller. Air was drawn from the chamber by a small diaphragm pump, the flow from the pump being controlled by a second mass flow controller.…”

“…[31] Deployment of the chamber on top of a column of sand through which CO 2 of a known isotopic signature was diffusing showed that the stable isotope values obtained with this system were representative of the source gas. [46] This system has recently been further developed and expanded to allow simultaneous deployment of four chambers (Fig. 4).…”

“…Ohlsson et al [59] have suggested that in closed chambers starting to make measurements from a low [CO 2 ] can encourage the ingress of atmospheric air and change the diffusion gradient. Online measurements using this system do tend to be initially enriched [46] so that in more recent work no pre-flushing was used. As with more classic open-chamber systems, because the [CO 2 ] above the soil surface remains close to ambient levels, long deployment times of an hour or more allow time for steady-state measurement conditions to develop and offer an opportunity to measure d 13 CR S accurately.…”