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/Al-SOC) 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 Fe- 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.Key words: karst soil, soil organic carbon, labile organic carbon, recalcitrant organic carbon, calcium-bound organic carbon, iron/aluminium-bound organic carbon.Kvantifikacija in vrednotenje organskega ogljika in njegovih frakcij v tleh: primer s Klasicnega Krasa, JZ SlovenijaZ meritvami organskega ogljika v prsteh (SOC) izražamo vsebnost organskih snovi v prsteh (SOM). SOM igra kljucno vlogo pri ekosistemskih storitvah, rodovitnosti, zmogljivosti zadrževanja vode in kroženju ogljika. SOC lahko razdelimo na razlicne ogljikove frakcije, ki kažejo razlicno stabilnost in kemicno sestavo, na katero razlicno vplivajo litoloska zgradba, bioloski in podnebni procesi. Boljse razumevanje SOC in vpliv razlicnih vrst maticne podlage na ogljikove frakcije bi lahko olajsala oceno stabilnosti SOC. Ta studija preucuje koncentracije in dinamiko razlicnih frakcij SOC (npr. labilni organski ogljik, LOC; rekalcitrantni organski ogljik, ROC; organski ogljik, vezan na kalcij; organski ogljik, vezan na železo/aluminij, Fe/Al-SOC) v gozdnih prsteh in povezanost z razlicno litolosko zgradbo v podobnih podnebnih razmerah. Prostorsko se studija osredotoca na obmocje klasicnega krasa. Koncentracija SOC in njegove frakcije so se z narascajoco globino v vseh profilih zmanjsale, kar kaže na relativno uravnotežene profile prsti. Vrednosti SOC (9,7–45,5 g∙kg-1) so skladne z ugotovitvami drugih raziskav prsti v regiji. ROC in Fe/Al-SOC (51,5–65,8 % oziroma 68,0–73,3 %) sta bili glavni komponenti SOC, medtem ko je za Ca-SOC znacilen bistveno nižji delež (6,4–7,4 %). Kljucna dejavnika, ki vplivata na vsebnost SOC, sta vsebnost kalcita (izražen kot kalcijev oksid) in gline; tadva s SOC tvorita mineralne komplekse, ki stabilizirajo SOC. Vrednosti Fe2O3 in Al2O3 ne pojasnjujejo razlik SOC niti njegovih frakcij, kar je lahko posledica razlicnega vpliva kemicnih/mineralnih oblik železovih oz. aluminijevih spojin na reaktivnost. Prsti na karbonatnih kamninah, ki so bogatejse z delci velikosti glin in CaO, so imele zaradi visjih koncentracij stabiliziranih SOC frakcij za 6,35 g∙kg-1 (28 odstotnih tock) visje povprecne vrednosti SOC v primerjavi prstmi na siliciklasticnih (flisnih) kamninah. Rezultati kažejo, da sta osnovna dejavnika, ki vplivata na stabilizacijo SOM, litoloska zgradba in pedogeneza.Kljucne besede: kraske prsti, organski ogljik v tleh, labilni organski ogljik, rekalcitrantni organski ogljik, organski ogljik, vezan na kalcij, organski ogljik, vezan na železo/aluminij.