Challenges in measuring the δ13C of the soil surface CO2 efflux

The delta C-13 of the soil surface efflux of carbon dioxide (delta(CRS)-C-13) 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. delta(CRS)-C-13 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 CO2 in order to measure delta(CRS)-C-13. 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). delta(CRS)-C-13 is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate delta(CRS)-C-13, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLS and CRDS analysis systems have opened up the possibility of measuring delta(CRS)-C-13 in real time permitting new research opportunities and are on balance the most suited to this type of measurement. Copyright (C) 2010 John Wiley & Sons, Ltd.