The role of (in)organic carbon removal as a pretreatment in the 13C isotope composition of various soils

Soil carbon content is a crucial property in fertility and soil health. Carbon occurs in the soil in organic and inorganic forms. Soil organic carbon enters the soil mainly as plant residues and root extracts from the biosphere, whereas the primary source of inorganic carbon is the parent material. Both organic and inorganic carbon forms are affected by the current environmental and artificial conditions and, consequently, change in space and time. In this dynamic system, biogen or pedogen effects can reform carbonates and change organic matter composition. Both carbon forms are the focus of research in the soil however, the interaction between them is less understood. Inorganic carbon forms play an essential role in soil organic matter stabilization and, therefore, in maintaining soil functions. The present study aimed to investigate the organic-inorganic carbon interaction processes in various soil horizons. As a proxy, the stable isotope δ¹³C values were measured separately in the organic and inorganic pools and the bulk soil. Altogether, 55 soil samples were taken from top and subsoils under various land uses and texture classes. Beyond the measurement of the bulk soils, one aliquot was heated to 550°C for six hours to eliminate the organic carbon content, whereas another one was treated with HCl to remove the carbonates. All samples were measured using a Thermo Scientific FLASH 2000 HT elemental analyzer mass spectrometer, identifying δ¹³C composition. The sum of organic and inorganic carbon was higher than the total carbon content (R²=0.94), suggesting that at least one carbon removal pretreatment was incomplete. Some low inorganic carbon content (<0.5%) revealed a deficient δ¹³C value (-15 – -30 ‰), indicating organic carbon residue presence. However, other slightly carbonated samples parallel with higher inorganic carbon content ones are in the range of pedogenic carbonates (0 – -10 ‰). The reason for the incomplete organic carbon removal is probably related to the organic-mineral complexes; however, further investigations are needed for a more convincing result. This work was supported by the National Research, Development and Innovation Fund of Hungary [project no. 2019-2.14-ERA-NET-2022-00037 and FK 142936]. Project no. 2019-2.14-ERA-NET-2022-00037 has been implemented with the support provided by the Ministry of Culture and Innovation of Hungary from the National Research, Development and Innovation Fund, financed under the ERA-NET COFUND/EJP COFUND funding scheme with co-funding from the European Union Horizon 2020 research and innovation programme.