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

RATIONALE Pyrogenic carbon ( CP) is an important component of the global carbon budget. Accurate determination of the abundance and stable isotope composition of CP in soils and sediments is crucial for understanding the dynamics of the CP 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 ( CL) from total organic carbon ( CT) in a range of matrices in order to enable reliable quantification of both the CP component of CT and the stable carbon isotope composition of CP ( δ13CP). METHODS We mixed CP at a range of concentrations with common CP-free matrices ( CL = cellulose, chitin, keratin, decomposed wood, leaf litter, grass and algae) and determined the amount of residual carbon not removed by hydrogen pyrolysis ( CR) as a ratio of CT ( CR/ CT). Mixing CP with a unique δ13C value provided a natural abundance isotope label from which to precisely determine the ratio of CP to residual CL remaining after hypy. RESULTS All CP-free matrices contained trace carbon after hypy, indicating that hypy does not remove all the CL. However, there was a strong correlation between CR/ CT and CP/ CT, viz. CR/ CT = 1.02( CP/ CT) + 4.0 × 10-3, r2 = 0.99, p <0.001, suggesting that only a small and reasonably constant fraction of CL remains after hypy. Uncertainties associated with the correction for contamination of CR by residual CL are minimal allowing for reliable determinations of both CP and δ13CP in many cases. CONCLUSIONS Hydrogen pyrolysis appears to be a robust technique for estimating CP abundance and δ13 CP across a range of materials. Nevertheless, caution is required in interpreting δ13 CP values when CP/ CT is low, with CP/ CT >4% being required for the determination of the δ13 CP values within an interpretable error under our experimental conditions. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]