A field-based method for simultaneous measurements of the δ<sup>18</sup>O and δ<sup>13</sup>C of soil CO<sub>2</sub> efflux

Abstract

Abstract. Three approaches for determining the stable isotopic composition (δ 13 C and δ 18 O) of soil CO 2 efflux were compared. A new technique employed mini-towers, constructed of open-topped piping, that were placed on the soil surface to collect soil-emitted CO 2 . Samples were collected along a vertical gradient and analyzed for CO 2 concentration and isotopic composition. These data were then used to produce Keeling plots to determine the δ 18 O and δ 13 C of CO 2 emitted from the soil. These results were then compared to the δ 18 O and δ 13 C of soil-respired CO 2 measured with two other techniques: (1) flux chambers and (2) estimation from the application of the diffusional fractionation factor to measured values of below ground soil CO 2 and to CO 2 in equilibrium with soil water δ 18 O. Mini-tower δ 18 O Keeling plots were linear and highly significant (0.81<r 2 <0.96), in contrast to chamber δ 18 O Keeling plots, which showed significant curvature, necessitating the use of a mass balance to calculate the δ 18 O of respired CO 2 . In the chambers, the values determined for the δ 18 O of soil respired CO 2 approached the value of CO 2 in equilibrium with surficial soil water, and the results were significantly δ 18 O enriched relative to the minitower results and the δ 18 O of soil CO 2 efflux determined from soil CO 2 . There were close agreements between the three methods for the determination of the δ 13 C of soil efflux CO 2 . Results suggest that the mini-towers can be effectively used in the field for determining the δ 18 O and the δ 13 C of soil-respired CO 2 .

Challenges in measuring the δ13C of the soil surface CO2 efflux

“…[65,69] Mini-towers are typically 1-2 m high columns which are open at the top and fitted with sample ports at regular spaced intervals. They are either .…”

“…inserted into the soil [69] or remain on the surface [65] and CO 2 from the soil surface diffuses up the tower. Gas samples are then collected from the ports and Keeling plots used to estimate d 13 CR S .…”

Application of a laser-based spectrometer for continuous in situ measurements of stable isotopes of soil CO&lt;sub&gt;2&lt;/sub&gt; in calcareous and acidic soils

“…The lack of a proper understanding of the diurnal fluctuations in soil CO 2 release might introduce uncertainty in estimating the soil carbon budget and the CO 2 fluxes to the atmosphere. The isotopic composition of soil CO 2 and its diel fluctuation can be a critical parameter for the partitioning of ecosystem gas exchange into its components (Bowling et al, 2003;Mortazavi et al, 2004) and for disentangling plant and ecosystem processes (Werner and Gessler, 2011). By assessing the δ 13 C of soil CO 2 , it is possible to identify the source for CO 2 (Kuzyakov, 2006) and the coupling between photosynthesis and soil respiration when taking into account postphotosynthetic isotope fractionation (Werner et al, 2012;Wingate et al, 2010).…”

“…The conventional method for estimating the δ 13 C and δ 18 O of soil CO 2 efflux is by using two end-member mixing models of atmospheric CO 2 and CO 2 produced in the soil (Keeling, 1958). The conventional methods for sampling soil produced CO 2 are chamber-based (Bertolini et al, 2006;Torn et al, 2003), "mini-tower" (Kayler et al, 2010;Mortazavi et al, 2004), and soil-gas-well-based (Breecker and Sharp, 2008;Oerter and Amundson, 2016) methods. In conventional methods, air sampling is done at specific time intervals, and δ 13 C and δ 18 O are analysed using isotope ratio mass spectrometry (IRMS; Ohlsson et al, 2005).…”

“…In chamber-based systems, nonsteady-state conditions may arise within the chamber due to increased CO 2 concentrations, which in turn hinders the diffusion of 12 CO 2 more strongly than that of heavier 13 CO 2 (Risk and Kellman, 2008). Moreover, it has been found that δ 18 O of the CO 2 inside a chamber is significantly influenced by the δ 18 O of the surface soil water, as an equilibrium iso-topic exchange happens during the upward diffusive movement of soil CO 2 (Mortazavi et al, 2004). The advent of laser-based isotope spectroscopy has enabled cost-effective, simple, and high precision real-time measurements of δ 13 C and δ 18 O in CO 2 (Kammer et al, 2011;Kerstel and Gianfrani, 2008).…”