Parent Material Shapes Carbon Quantity and Composition Across Soil Profiles: Insights from MAOM and POM Fractionation

Soils hold the largest terrestrial reservoir of organic carbon (OC), and understanding the factors that are essential for soil OC formation, stabilization and transformation is crucial for predicting carbon-climate feedbacks. Mineral-associated organic matter (MAOM) is widely recognized as a dominant stabilization pathway for soil OC. However, the extent to which mineralogy - and the elemental makeup of mineral assemblages - regulates MAOM formation and persistence remains insufficiently resolved. Here we compare OC composition of six soil profiles developed on contrasting parent materials (sandstone, shale, and dolomite) in the Sudeten Mountains of central Europe. Bulk soil was fractionated into matrix-free particulate organic matter (POM), and MAOM using combined density and grain size separation.

In shale- and dolomite-hosted profiles, MAOM accounts for the majority weight, whereas sandstone-hosted profiles contain a greater proportion of POM. With depth, the relative weight contribution of MAOM decreases and POM increases in all profiles. Regarding the total organic carbon (TOC), sandstone-derived profiles generally contain less OC than shale- and dolomite-derived soils. TOC declines with depth in each fraction, with consistently higher OC content in MAOM than in POM. Similar with the weight distribution, the relative OC contribution of MAOM decreases while POM increases, despite the overall OC depletion down-profile.

To probe stabilization mechanisms beyond concentration patterns, ramped pyrolysis/oxidation (RPO) is being applied to quantify thermal stability of OC among mineralogies. A positive correlation between TOC and average activation energy presents in most profiles except for one sandstone-based profile. This is contrary to the general expectation that low-TOC samples should contain higher average activation energy (i.e., a negative TOC-stability relationship). The positive correlation in our samples suggests that specific compound classes or organo–mineral associations, rather than OC content alone, may drive apparent thermal stability; ongoing FT-ICR-MS will target compositional drivers. In addition, the standard deviation of energy decreases down-profile, indicating a decrease in OC heterogeneity.

To enable cross-mineral comparison, the OC loading of different fractions is also being normalized to mineral specific surface area (SSA). Upcoming ICP-OES measurements of elemental composition will test links between MAOM abundance and key elements, with the goal of identifying mineralogical and elemental controls on MAOM stabilization across contrasting parent materials.