Quantifying the abundance and stable isotope composition of pyrogenic carbon using hydrogen pyrolysis

RATIONALE Pyrogenic carbon ( C P ) is an important component of the global carbon budget. Accurate determination of the abundance and stable isotope composition of C P in soils and sediments is crucial for understanding the dynamics of the C P cycle and interpreting records of biomass burning, climate and vegetation change in the past. Here we test hydrogen pyrolysis (hypy) as a new technique potentially capable of eliminating labile organic carbon ( C L ) from total organic carbon ( C T ) in a range of matrices in order to enable reliable quantification of both the C P component of C T and the stable carbon isotope composition of C P ( δ 13 C P ). METHODS We mixed C P at a range of concentrations with common C P ‐free matrices ( C L  = cellulose, chitin, keratin, decomposed wood, leaf litter, grass and algae) and determined the amount of residual carbon not removed by hydrogen pyrolysis ( C R ) as a ratio of C T ( C R / C T ). Mixing C P with a unique δ 13 C value provided a natural abundance isotope label from which to precisely determine the ratio of C P to residual C L remaining after hypy. RESULTS All C P ‐free matrices contained trace carbon after hypy, indicating that hypy does not remove all the C L . However, there was a strong correlation between C R / C T and C P / C T , viz . C R / C T  = 1.02( C P / C T ) + 4.0 × 10 –3 , r 2  = 0.99, p <0.001, suggesting that only a small and reasonably constant fraction of C L remains after hypy. Uncertainties associated with the correction for contamination of C R by residual C L are minimal allowing for reliable determinations of both C P and δ 13 C P in many cases. CONCLUSIONS Hydrogen pyrolysis appears to be a robust technique for estimating C P abundance and δ 13 C P across a range of materials. Nevertheless, caution is required in interpreting δ 13 C P values when C P / C T is low, with C P / C T >4% being required for the determination of the δ 13 C P values within an interpretable error under our experimental conditions. Copyright © 2012 John Wiley & Sons, Ltd.