Depth effects of organic matter stabilization in temperate acidic forest soils

Soil organic matter (SOM) is comprised of free and occluded particulate organic matter (fPOM and oPOM) and mineral-associated organic matter (MAOM). Understanding of the mechanisms underlying the formation of these distinct pools and their contributions to SOM stabilization remains limited, particularly along soil horizons with increasing depth. This research aims to clarify the factors controlling stabilization of these SOM pools, and their molecular composition in topsoil and subsoil horizons. To achieve these objectives, topsoil (A horizons; 0–10/15 cm) and subsoil (B horizons; 10/15–30 cm) were collected from 14 temperate acidic forest soils (pH 3.8–5.3) to generate a gradient of active Al/Fe (oxalate- and pyrophosphate-extractable Al and Fe) and C-Al/Fe saturation. Physical fractionation followed by analytical pyrolysis showed higher proportions of MAOM (72% of SOM) and a larger abundance of highly-degraded compounds (e.g., N-containing compounds) (72%) in subsoil, highlighting its more stable and processed nature compared to topsoil. The increase in bulk C content in topsoil was largely driven by oPOM (28%), with aggregation facilitated by a larger clay + silt content compared to subsoil. The positive correlation between oPOM and MAOM C contents and long-chain n-alkenes suggests the importance of plant-derived materials and aggregate hydrophobicity for SOM stabilization in topsoil. By contrast, subsoil C was primarily composed of both oPOM (21% of SOM) and MAOM (72%), with stabilization through oPOM controlled by active Al/Fe content rather than clay + silt. Furthermore, oPOM formation in subsoil mainly originated from microbial N-containing compounds. Overall, our study highlights that clay + silt in topsoil and active Al/Fe in subsoil contribute decisively to SOM stabilization, with long-chain lipids and N-containing compounds driving the formation of stable SOM pools with increasing soil depth.