Challenges in measuring the δ¹³C of the soil surface CO₂ efflux

Abstract

The d 13C of the soil surface efflux of carbon dioxide (d 13 CR S ) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. d 13 CR S can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO 2 in order to measure d 13 CR S . Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS Increasing levels of atmospheric carbon dioxide (CO 2 ) and predicted climate change have intensified research efforts to better understand the global carbon (C) cycle. Within terrestrial ecosystems, soils hold substantial amounts of C and through the exchange of CO 2 with the atmosphere have the ability to act as either a C source or a sink.[1] It is this balance and how it may be influenced by future climate change and/or land use change which has been the focus of considerable debate for some time.[2-5] The net exchange of CO 2 across a landscape can be measured using a specialist technique called eddy covariance.[6] However, eddy covariance measurements do not provide any detailed information on the underlying mechanisms which drive the terrestrial C cycle, namely photosynthesis and respiration. [7] For several decades now the stable carbon isotope 13 C has been used extensively to provide a better understanding of these two processes at a range of scales. At the ecosystem scale so-called isofluxes [8] can be measured by combining micrometeorological measurements with the 13 C analysis of CO 2 within and above the plant canopy. This type of measurement allows total ecosystem respiration to be measured -a combination of both plant (stem/leaf) respiration and the respiration from soil, or efflux rate. With the contribution of soil-derived CO 2 being important as this can provide information on the longterm overall net C balance of the system. Two basic processes contribute to the soil surface CO 2 efflux rate (R S ): autotrophic respiration from roots and the rhizosphere, and heterotrophic respiration from the turnover of soil organic matter (SOM).Against this background, measuring the rate at which CO 2 leaves the soil surface coupled with the measurement of its 13 C/ 12 C ratio (d 13 CR S ) has emerged as a key tool in ecophysiological research and global terrestrial C cycle studies. Isotopic analyses provide a means of distinguishing the C sources which contribute to this overall flux, allowing either the autotrophic or the heterotrophic components to be quantified. For example, d13 CR S measurements have been used to assess the speed at which assimilated C is transfe...

Field-based cavity ring-down spectrometry of δ13C in soil-respired CO2

“…Furthermore, the difference between the δ 13 C value of soil CO 2 and respired CO 2 , which can be up to 4.4 ‰, is altered by the CO 2 production rate and is rarely at steady state [15]. The complexity of the diffusion dynamics of soil CO 2 and its influence on the δ 13 C value of soilrespired CO 2 means that the 'true' value of the isotopic composition of the CO 2 efflux is usually unknown.…”

“…The kinetic fractionation of 12 C/ 13 C in CO 2 as the gas diffuses through the soil to the atmosphere leads to 13 C enrichment in soil CO 2 relative to the CO 2 respired at the surface of the soil [15]. Furthermore, the difference between the δ 13 C value of soil CO 2 and respired CO 2 , which can be up to 4.4 ‰, is altered by the CO 2 production rate and is rarely at steady state [15].…”

“…[7][8][9][10][11][12][13][14]. Recently, Midwood and Millard [15] reviewed a range of techniques and found that CO 2 efflux measurements are substantially influenced by the soil chamber design and that most designs have some limitations affecting measurement accuracy under certain conditions. In particular, small pressure differences between the inside and outside of the soil chamber can have a pronounced effect on the rate of CO 2 efflux [14].…”

“…In particular, small pressure differences between the inside and outside of the soil chamber can have a pronounced effect on the rate of CO 2 efflux [14]. As a result of diffusion-induced kinetic fractionation of 12 C/ 13 C between CO 2 within the soil and CO 2 efflux from the soil surface, method-dependent variations in CO 2 efflux may also alter the δ 13 C value of the sampled CO 2 [15].…”

System for <i>δ</i><sup>13</sup>C–CO<sub>2</sub> and <i>x</i>CO<sub>2</sub> analysis of discrete gas samples by cavity ring-down spectroscopy

“…For example, simultaneous highprecision isotope ratio and mole fraction measurements from isotopic-CO 2 CRDS will reduce empirical workload and in-crease accuracy of CO 2 flux partitioning calculations in soil and plant respiration experiments (Midwood and Millard, 2011;Snell et al, 2014). However, realising these benefits requires regular batch analysis of discrete samples -existing arrangements that couple CRDS instruments directly to soil headspace chambers are generally constrained to measuring just one experiment at a time (Albanito et al, 2012;Bai et al, 2011;Midwood et al, 2008).…”