Collection and storage of CO 2 for 13 C analysis: an application to separate soil CO 2 efflux into root‐ and soil‐derived components

Soil surface CO 2 efflux is comprised of CO 2 from (i) root respiration and rhizosphere microbes and (ii) heterotrophic respiration from the breakdown of soil organic matter (SOM). This efflux may be partitioned between these sources using δ 13 C measurements. To achieve this, continuous flow isotope ratio mass spectrometry can be used and, in conjunction with 10 mL septum‐capped vials, large numbers of samples may be analysed using a Finnigan MAT Delta plus XP interfaced to a Gas Bench II. Here we describe a number of advances to facilitate such work, including: (i) a technique for monitoring mass spectrometer performance, (ii) improvements to sample storage, and (iii) a gas‐handling system for incubating and sampling the CO 2 derived from roots and soils. Mass spectrometer performance was monitored using an automated refillable vial. Compressed air analysed with this system had mean δ 13 C of −9.61 ± 0.16‰ (± 1 σ , n = 28) collected over four runs. Heating the butyl rubber septa used to seal the vials at 105°C for 12 h improved the sample storage. After air transportation over 12 days, the isotope composition of the CO 2 at ambient concentrations was unchanged (before: −35.2 ± 0.10‰, n = 4; after: −35.3 ± 0.10‰, n = 15); without heat treatment of the septa the CO 2 became slightly enriched (–35.0 ± 0.14‰, n = 15). The linearity of the Gas Bench II was found to decline above 8000 µmol CO 2 mol –1 . To stay within a linear range and to allow the incubation of soil and root material we describe a gas‐handling system based around a peristaltic pump. Finally, we demonstrate these methods by growing a C‐4 grass (Guinea grass, Panicum maximum Jacq.) in a C‐3 soil. Root respiration was found to contribute between 5 and 22% to the soil surface CO 2 efflux. These methodologies will facilitate experiments aimed at measuring the isotopic composition of soil‐derived CO 2 across a range of ecological applications. Copyright © 2006 John Wiley & Sons, Ltd.