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 (δ¹³C) are used to assess the C origin of SOC or SIC. However, as measuring SOC and SIC contents, measuring δ¹³C_SOC and δ¹³C_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 δ¹³C_SOC and δ¹³C_SIC. We coupled a RE device to an isotopic gas analyser (Picarro) to continuously measure δ¹³C_CO2 and approach δ¹³C_SOC and δ¹³C_SIC. We hypothesised that different carbonate mineralogies and/or crystal sizes in SIC involve fluctuations of the δ¹³C_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 δ¹³C_CO2 values were obtained before and after 650°C and were consistent with the δ¹³C_SOC and δ¹³C_SIC obtained by EA-IRMS. The fluctuations of δ¹³C_CO2 above 650°C were higher with calcite/shell mixes than with pure carbonates. A δ¹³C_CO2 fluctuation > ± 0.2‰ could be a pertinent indicator to detect mixes of carbonate with different δ¹³C in soils. The RE-Picarro setup is promising to assess SOC and SIC contents, δ¹³C_SOC and δ¹³C_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.