Minimising methodological biases to improve the accuracy of partitioning soil respiration using natural abundance 13C

• Published in: Rapid communications in mass spectrometry Vol. 28; no. 21; pp. 2341 - 2351
• Main Authors: Snell, Helen S. K., Robinson, David, Midwood, Andrew J.
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 15-11-2014; Wiley Subscription Services, Inc
• Subjects: Carbon Dioxide - analysis; Carbon Dioxide - metabolism; Carbon Isotopes - analysis; Ecosystem; Mass Spectrometry - methods; Scotland; Soil - chemistry
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.7017

RATIONALE
Microbial degradation of soil organic matter (heterotrophic respiration) is a key determinant of net ecosystem exchange of carbon, but it is difficult to measure because the CO2 efflux from the soil surface is derived not only from heterotrophic respiration, but also from plant root and rhizosphere respiration (autotrophic). Partitioning total CO2 efflux can be achieved using the different natural abundance stable isotope ratios (δ13C) of root and soil CO2. Successful partitioning requires very accurate measurements of total soil efflux δ13CO2 and the δ13CO2 of the autotrophic and heterotrophic sources, which typically differ by just 2–8 ‰.
METHODS
In Scottish moorland and grass mesocosm studies we systematically tested some of the most commonly used techniques in order to identify and minimise methodological errors. Typical partitioning methods are to sample the total soil‐surface CO2 efflux using a chamber, then to sample CO2 from incubated soil‐free roots and root‐free soil. We investigated the effect of collar depth on chamber measurements of surface efflux δ13CO2 and the effect of incubation time on estimates of end‐member δ13CO2.
RESULTS
(1) a 5 cm increase in collar depth affects the measurement of surface efflux δ13CO2 by –1.5 ‰ and there are fundamental inconsistencies between modelled and measured biases; (2) the heterotrophic δ13CO2 changes by up to −4 ‰ within minutes of sampling; we recommend using regression to estimate the in situ δ13CO2 values; (3) autotrophic δ13CO2 measurements are reliable if root CO2 is sampled within an hour of excavation; (4) correction factors should be used to account for instrument drift of up to 3 ‰ and concentration‐dependent non‐linearity of CRDS (cavity ringdown spectroscopy) analysis.
CONCLUSIONS
Methodological biases can lead to large inaccuracies in partitioning estimates. The utility of stable isotope partitioning of soil CO2 efflux will be enhanced by consensus on the optimum measurement protocols and by minimising disturbance, particularly during chamber measurements. Copyright © 2014 John Wiley & Sons, Ltd.