Combining Density Fractionation and Ca K-Edge XANES Reveals Contrasting SOM–Mineral Association Patterns Shaped by Parent Material in Forest Soil O Horizons

Abstract

Background: Soil organic matter (SOM) accumulation in forest soils is strongly influenced by interactions with inorganic constituents derived from parent material mineralogy. However, their role in the forest floor remains underexplored. Aims and methods: This study investigates how different bedrock types (basalt, paragneiss, dolomite, limestone) influence SOM–mineral associations in organic (Of, Oh) and mineral (Ah) horizons of temperate forest soils, exploring the role of Ca in soil organic carbon (SOC) accumulation. We applied sequential density fractionation to separate SOM into low- (<1.4 g cm−3, SOM–metal cation complexes), intermediate- (1.4–1.6 g cm−3, SOM–metal complexes and mineral associations), and heavy-density fractions (>1.6 g cm−3, mineral-associated SOM). We quantified SOC, total Al, Fe, Ca, and Mg, and identified Ca species using Ca K-edge x-ray absorption near-edge structure (XANES) spectroscopy. Results: In organic layers from calcareous bedrock, SOM–metal cation complexes—particularly Ca-organic complexes (e.g., Ca-carboxylates, Ca-phenols, Ca-phytates)—dominated in low- and intermediate-density fractions, likely driven by enhanced Ca2+ availability. This was associated with higher SOC concentrations (Of: 487–451 mg g−1; Oh: 458–415 mg g−1). In contrast, silicate soils—especially basalt—showed higher inorganic material contents (Of: 19%, Oh: 43%) and larger mineral-associated SOM pools (Of: 13%, Oh: 38%), likely due to intensive bioturbation and abundant pedogenic Al and Fe minerals. Ca-XANES revealed that Ca-bearing mineral types in O layers strongly depended on bedrock composition and soil pedogenesis. Conclusions: This study highlights the role of parent material in shaping SOM–mineral associations and interactions, underscoring the importance of Ca in SOC accumulation on the forest floor.