Methodological comparison for quantitative analysis of fossil and recently derived carbon in mine soils with high content of aliphatic kerogen
Abstract
In mine soil, quantification of soil organic carbon (OC) derived recently from biomass decomposition is complicated by the presence of fossil (geogenic) C derived from coal, oil shale, or similar material in the overburden. The only reliable method for such measurement is C-14 analysis (i.e. radiocarbon dating) using instrumentation such as accelerator mass spectrometry, which is too expensive for routine laboratory analysis.
We tested two previously used and two new methods for recent C quantification and compared them with C-14 AMS radiocarbon dating as a reference using a set of soil samples (n = 14) from Sokolov, Czech Republic: (i) C-13 isotope ratio composition, (ii) cross polarization magic angle spinning C-13 nuclear magnetic resonance (CPMAS C-13 NMR) spectroscopy, (iii) near infrared spectroscopy (NIRS) coupled with partial least squares regression and (iv) Rock-Eval pyrolysis. Conventional methods for OC determination (dry combustion, wet dichromate oxidation, loss-on-ignition) were also compared to quantify any bias connected with their use.
All the methods provided acceptable recent carbon estimates in the presence of mostly aliphatic fossil C from kerogen. However, the most accurate predictions were obtained with two approaches using Rock-Eval pyrolysis parameters as predictors, namely (i) S-2 curve components and (ii) oxygen index (OI).
The S-2 curve approach is based on the lower thermal stability of recent vs. fossil organic matter. The OI approach corresponded well with C-13 NMR spectra, which showed that samples rich in recent C were richer in carboxyl C and O-alkyl C.
These two methods showed the greatest potential as routine methods for recent C quantification.