Exploring Bedrock-Driven SOM Dynamics and SOM-Mineral Associations in Forest Soil Organic Surface Layers

Soil organic matter (SOM) in temperate forests starts its formation on the forest floor and supports essential ecosystem functions. Understanding its composition and dynamics is crucial for sustainable forest management. SOM accumulation depends strongly on interactions with mineral compounds, which protect it from microbial decomposition through various mechanisms. Our previous research using multi-step density fractionation, chemical characterization (main cations, total carbon), and Ca-XANES spectroscopy demonstrated that bedrock type (basalt, paragneiss, dolomite, limestone) significantly influences SOM-mineral associations and binding patterns in the organic soil surface layers (Of, Oh) of beech-dominated temperate forests.

This study further examines SOM composition, focusing on functional C groups (alkyl, O-alkyl, aryl, carboxyl) of bulk soils and density fractions of organic layers from different bedrock types using Cross-polarization (CP) magic angle spinning (MAS) ¹³C NMR spectroscopy. We also assess biomolecular composition (carbohydrates, carbonyls, lipids, lignin, proteins, and char) through the application of the Molecular Mixing Model by Nelson & Baldock (2005) and analyze non-cellulosic polysaccharides via gas chromatography to identify primary (plant-derived) and secondary (microbial-derived) polysaccharides.

Preliminary results reveal that Of layers are primarily composed of undecomposed or partially decomposed plant-derived SOM, with light fractions (<1.6 g cm⁻³) accounting for most SOM mass, enriched in O-alkyl compounds (e.g., carbohydrates, lignin). Despite the heavy fraction (>1.6 g cm⁻³), representing mineral-associated SOM, is small in Of layers, basalt samples had the largest contribution overall. This fraction contained more proteins and lipids, indicating advanced microbial processing. The marked carbonyl/carboxyl accumulation in basalt soils suggested enhanced stabilization via carboxylate sorption to Fe and Ca mineral surfaces. In Oh layers, density fractions >1.4 g cm⁻³ dominate, reflecting increased OM stabilization and decomposition. Bedrock-specific effects include elevated lipid accumulation in paragneiss soils and higher carbonyl/carboxyl and char ̶ fire-derived OM in dolomite soils. Higher Alkyl/O-Alkyl ratios in silicate-derived soils (e.g., paragneiss) indicate advanced SOM decomposition, accompanied by increased microbial-derived polysaccharide contributions (galactose, mannose), highlighting dynamic turnover in these soils.

Our findings highlight the interplay between SOM composition, SOM-mineral interactions, and bedrock type in regulating SOM dynamics. Future Fe-XANES analysis will clarify the role of different iron species (e.g., Fe-SOM complexes, different Fe minerals) in SOM stabilization and decomposition.