Comparison of soil organic carbon speciation using C NEXAFS and CPMAS ¹³C NMR spectroscopy

We compared synchrotron-based C near-edge X-ray absorption fine structure (NEXAFS) and CPMAS ¹³C nuclear magnetic resonance (NMR) spectroscopy with respect to their precision and accuracy to quantify different organic carbon (OC) species in defined mixtures of soil organic matter source compounds. We also used both methods to quantify different OC species in organic surface horizons of a Histic Leptosol as well as in mineral topsoil and subsoil horizons of two soils with different parent material, stage of pedogenesis, and OC content (Cambisol: 15–30 OC mg g⁻¹, Podzol: 0.9–7 OC mg g⁻¹). CPMAS ¹³C NMR spectroscopy was more accurate and precise (mean recovery of different C functional groups 96–103%) than C NEXAFS spectroscopy (mean recovery 92–113%). For organic surface and topsoil samples, NMR spectroscopy consistently yielded larger O-alkyl C percentages and smaller alkyl C percentages than C NEXAFS spectroscopy. For the Cambisol subsoil samples both methods performed well and showed similar C speciation results. NEXAFS spectroscopy yielded excellent spectra with a high signal-to-noise ratio also for OC-poor Podzol subsoil samples, whereas this was not the case for CPMAS ¹³C NMR spectroscopy even after sample treatment with HF. Our results confirm the analytical power of CPMAS ¹³C NMR spectroscopy for a reliable quantitative OC speciation in soils with >10 mg OC g⁻¹. Moreover, they highlight the potential of synchrotron-based C NEXAFS spectroscopy as fast, non-invasive method to semi-quantify different C functional groups in soils with low C content (0.9–10 mg g⁻¹).