QUANTIFICATION AND EVALUATION OF SOIL ORGANIC CARBON AND ITS FRACTIONS: CASE STUDY FROM THE CLASSICAL KARST, SW SLOVENIA

Soil organic carbon (SOC) is a critical measure of soil organic matter (SOM) content. SOM plays a vital role in ecosystem services, soil fertility, soil water retention capacity, and carbon cycling. SOC can be partitioned into various carbon fractions, which exhibit diverse stability and chemical compositions that are influenced variably by lithology as well as biological and climatic processes. A better understanding of SOC and the influence of different bedrock types on carbon fractions could facilitate the evaluation of the fate and stability of SOC. The present study is focused on the concentrations and characteristics of different SOC fractions (e.g., Labile organic carbon, LOC; Recalcitrant organic carbon, ROC; Calcium-bound organic carbon, Ca-SOC; Iron/aluminium-bound organic carbon, Fe/AlSOC) in forest soils associated with different bedrock lithology under similar climate conditions in the centre of the 'Classical Karst', and evaluates influence of the geological environment on SOC. SOC and SOC fraction concentrations decreased with an increase in depth in all profiles, indicating stabilized soil profiles. SOC values (9.7-45.5 g.kg(-1)) were consistent with the findings of other studies on soils in the region. ROC and Fe/Al-SOC (51.5-65.8 % and 68.0-73.3 %, respectively) were the major SOC fractions, while Ca-SOC accounted for a considerably lower proportion (6.4-7.4 %) of the SOC contents. Key factors influencing SOC contents were calcite (expressed as calcium oxide) and clay contents, which represent mineral complexes stabilizing SOC. Overall Fe2O3 and Al2O3 concentration did not explain differences in SOC nor its fractions, potentially due to the importance of chemical/mineral forms of Fc- and Al-related minerals (reactivity). Soils on carbonate rocks, which are richer in clay and CaO, had 6.35 g.kg(-1) (28 percentage points) higher concentrations of SOC average when compared with soils on siliciclastic rock, due to higher concentrations of stabilized SOC fractions. The results demonstrate that bedrock lithology and pedogenesis are key factors influencing SOM stabilization.