Organic matter content and features related to associated mineral fractions in an acid, loamy soil

Sorption of soil organic matter on mineral surfaces is now recognized as a major organic carbon (C) stabilization process. To investigate the role of organo‐mineral interactions in carbon‐rich soils, we separated organo‐mineral associations by sequential density fractionation in the top‐ and subsoil of a forest Acrisol (umbrept) from southern France. Fractions were characterized by scanning electron microscopy, X‐ray diffraction, elemental analysis, ¹³C NMR and δ¹³C. For each horizon, we isolated five types of association: light organic matter non‐occluded, and occluded in aggregates (<1.9 g cm⁻³), and microaggregates formed by poorly crystallized mineral phases (1.9–2.2 g cm⁻³), by phyllosilicates (2.2–2.5 g cm⁻³) and by primary mineral particles weakly complexed with organic compounds (>2.5 g cm⁻³). Carbon accumulation in organo‐mineral fractions was correlated with a relative enrichment in hydrophobic compounds (alkyl C or aromatic C). In contrast, the content in nitrogen and carboxylic groups was more constant. Poorly crystalline mineral phases stabilized the largest amount of soil organic matter. We compiled data from previous studies carried out on soils with different mineralogies (n = 20) and found a significant correlation between the δ¹³C and the C:N ratio of sorbed organic compounds according to the C content of fractions. This general trend is in line with the conceptual model of a zonal structure of soil organic matter sorbed on mineral surfaces. Furthermore, we hypothesized that the accumulation of hydrophilic (O‐alkyl C) and hydrophobic (alkyl C and/or aromatic C) organic compounds on mineral surfaces is favoured by the structural organization of organo‐mineral associations. The same organo‐mineral associations were found in top‐ and subsoils, indicating similar stabilization processes in surface and deeper horizons.