Analytical distinction between soil organic and inorganic carbon contents and 13C natural abundances: Evaluation of a heating pretreatment

Research on soil health, fertility, and climate change primarily focuses on soil organic carbon (SOC), while overlooking soil inorganic carbon (SIC), which is widespread in arid environments. Reliable analyses of SOC and SIC, along with their natural 13C isotopic abundances, are critical but remain questionable. The SOC and SIC are determined by applying a pretreatment to eliminate either of them. Acidification to remove SIC is a well-established approach for determining SOC; SIC is then calculated as the difference between total carbon (TC) and SOC. Determination of SIC by heating soil (550 °C) to remove SOC is less explored. The present study compared these two methods and their 13C content across 160 soils from three continents. Heating was acceptable for SOC- and SIC-rich soils, dominated by lithogenic SIC, with similar calculated and measured SIC and 13C-SIC values, suggesting that either method might be used. However, for soils with low SIC (<6 g kg−1 soil), high SOC/TC ratios (>0.6), and some pedogenic SIC, neither method was entirely satisfactory. Methodological development is needed for SIC-poor soils; however, because interactions between SOC and SIC challenged their strict separation, further research is required to define and quantify these specific interactions.