Quantifying and characterizing soil organic and inorganic carbon in calcareous soils

Recently, many literature reviews have focused on soil inorganic carbon (SIC) and concluded that SIC should be studied as much as soil organic carbon (SOC) to complete the soil C balance at global scale. However, as SIC has been rarely studied lately, analytical methods for measuring its carbon content and 13C in calcareous soils have not been refined. Similarly, the quantification of different SIC pools remained poorly assessed. This study aimed to illustrate the diversity of SOC and SIC pools in calcareous soils. A soil collection of 160 soil samples were selected from mostly agricultural lands in Canada, Hungary, France, Germany, Italy, and Tunisia to cover a large range of SOC and SIC contents. The carbonate composition was mainly in the form of calcite and, to a lesser extent, dolomite. The natural abundance in ¹³C of SOC and SIC showed a small range of values for SOC (-35 to -20 ‰, median: -27‰) signalling a predominance of C3-plants, and a large range of values for the SIC (-31 to +3 ‰; median -5 ‰), indicating a possible mixture of inorganic carbon from different sources (Hazera et al. 2025). Standard procedures to quantify SOC and SIC involving pretreatments to remove one of the C forms and/or calculations could lead to both analytical errors and substantial measurement errors (Jakab et al. submitted). Thermal analysis (i.e. Rock-Eval®, RE) has been adapted to estimate SOC and SIC contents on a non-pretreated soil sample without needing statistical post-hoc corrections (Hazera et al., 2023, in press). This RE method was applied to characterize more than 400 particle-size fractions of 65 calcareous soils i.e., particulate organic matters (POM, > 50 µm) and mineral associated organic matters (MaOM, 20-50, 0-20 and 0-2 µm). The variability of C distribution in the different fractions was more pronounced for SIC than SOC: the SOC was mainly contained in the < 20 µm and POM fractions while the SIC was distributed in all the particle-size fractions. As expected, POM fractions presented less degraded organic matter than MaOM, with higher Hydrogen index and lower Oxygen index (POM: 305 ± 63 mgHC.g^-1TOC and 261 ± 37 mgO₂.g^-1TOC versus MaOM: 144 ± 56 mgHC.g^-1TOC and 400 ± 112 mgO₂.g^-1TOC). Signals obtained during SIC thermal breakdown were also examined to study the SIC forms in the soil fractions. SIC signals were comparable between soil fractions for some soils, but could also vary considerably for others, suggesting a diversity in either the mineralogy or the SIC forms distributed according to the particle-size soil fractions in these soils. These preliminary results need further investigations to identify the origin of SIC e.g. by studying the isotopic signature δ¹³C of SOC and SIC in the soil fractions, and to determinate their sensitivity to dissolution. This communication illustrates the need to better quantify the different forms of SIC to understand its dynamics and interactions with SOC.