Coupling Infrared Isotopic Gas Analysis and Thermal Ramped Analysis to Characterise Soil Organic and Inorganic Carbon

ABSTRACT Studying the soil organic and inorganic carbon (SOC and SIC) dynamics is essential to assess the carbon (C) sequestration potential of calcareous soils. Isotopic signatures (δ 13 C) are used to assess the C origin of SOC or SIC. However, as measuring SOC and SIC contents, measuring δ 13 C SOC and δ 13 C SIC on a non‐pretreated aliquot remains a challenge. Thermal analyses, like the Rock‐Eval (RE) analysis, are promising to quantify SOC and SIC in a single analysis, but, to our knowledge, no development was conducted to assess δ 13 C SOC and δ 13 C SIC . We coupled a RE device to an isotopic gas analyser (Picarro) to continuously measure δ 13 C CO2 and approach δ 13 C SOC and δ 13 C SIC . We hypothesised that different carbonate mineralogies and/or crystal sizes in SIC involve fluctuations of the δ 13 C CO2 . Two calcareous soils, a lithogenic (calcite) and a biogenic (snail shell) carbonate, and five calcite/shell mixes were analysed with the RE‐Picarro setup. Two distinct δ 13 C CO2 values were obtained before and after 650°C and were consistent with the δ 13 C SOC and δ 13 C SIC obtained by EA‐IRMS. The fluctuations of δ 13 C CO2 above 650°C were higher with calcite/shell mixes than with pure carbonates. A δ 13 C CO2 fluctuation > ± 0.2‰ could be a pertinent indicator to detect mixes of carbonate with different δ 13 C in soils. The RE‐Picarro setup is promising to assess SOC and SIC contents, δ 13 C SOC and δ 13 C SIC and detect mixes of carbonate with different origin on a non‐pretreated aliquot. Development is needed (i) on more soil and carbonate samples and (ii) to improve the precision and accuracy of the RE‐Picarro setup.