Pedogenic controls on organic carbon accumulation in Phaeozem and Histosol profiles of the Ciuc intramontane depression, Eastern Carpathians, Romania

Abstract

Abstract Intramontane depressions of the Eastern Carpathians host soils with substantial organic carbon enrichment, yet profile-based documentation of the pedogenic mechanisms involved remains limited. This study presents two contrasting soil profiles from the Ciuc intramontane depression: (1) a Cambic Hortic Chernic Phaeozem developed on a lower terrace of the Olt River, and (2) a Sapric Histosol located in the Olt floodplain. The Phaeozem exhibits a thick humic sequence extending to 80 cm depth, with soil organic carbon (SOC) exceeding 2% throughout the humic horizons. A darker subsurface chernic horizon, together with sedimentary and bioturbation features, is interpreted as evidence of polygenetic development involving successive phases of sediment accumulation and soil formation. Portable X-ray fluorescence (pXRF) analysis reveals relatively stable major oxide composition throughout the Phaeozem profile and does not indicate a sharp geochemical discontinuity across the humic–cambic boundary, suggesting that the observed textural differentiation reflects the influence of granulometrically differentiated terrace sediments subsequently modified by pedogenic processes. The Histosol contains a 55 cm thick sapric peat sequence with SOC exceeding 17%, underlain by mineral sediments. pXRF reveals unusually high calcium concentrations in the peat horizons (CaO: 7–13%), combined with low carbonate contents (1–2% CaCO₃) and near-neutral pH (7.2–7.8), characteristic of a calcium-rich minerotrophic fen. The discrepancy between pXRF-derived CaO and CaCO₃ contents, after conversion to elemental Ca equivalents, suggests that a large proportion of the pXRF-derived Ca signal is not accounted for by the measured carbonate equivalent, although its specific forms cannot be resolved from the available data. Together, these profiles document two contrasting expressions of SOC enrichment and organic matter preservation: humic-sequence development in a polygenetic mineral soil and peat preservation in a minerotrophic floodplain setting. The comparison suggests that geomorphic position, drainage conditions, sedimentary context and base-rich hydrological conditions jointly shape SOC patterns in these intramontane soils. The study provides a profile-based pedological interpretation of two contrasting soil-forming environments in the Ciuc intramontane depression.