Influence of Clay Mineralogy on Organic Matter Stabilization Potential

Clay minerals are recognized for their capacity to protect organic matter (OM) from microbial degradation, thereby playing a significant role in soil carbon sequestration. However, recent studies have yielded conflicting results regarding the maximum saturation limit of mineral-associated organic carbon in agricultural soils, highlighting the need to better understand the underlying fine-scale processes.

This study aimed to investigate the maximum capacity of different clay minerals to stabilize OM and to evaluate the saturation limits of mineral surfaces. A controlled laboratory experiment was conducted using three clay types with distinct specific surface areas: kaolinite, montmorillonite, and sepiolite. Microcosms were prepared using a sand-clay mixture (80% sand, 20% clay) with increasing proportions of green waste compost (GWC) at 1%, 5%, 10%, 25%, and 50%. Weekly CO₂ emissions were monitored over six months. At the conclusion of the incubation period, interactions between clay minerals and OM were examined via scanning electron microscopy (SEM). Bacterial and fungal abundances were quantified using quantitative PCR (qPCR), and microbial catabolic activity was assessed with Biolog EcoPlates™. Furthermore, OM thermal stability was evaluated using Rock-Eval® pyrolysis, while biological stability was assessed through the temperature sensitivity of microbial respiration (Q₁₀).

CO₂ emission data indicated the lowest release in treatments with sepiolite, followed by montmorillonite and kaolinite, with the highest emissions observed in control treatments without clay minerals. The extensive specific surface area of sepiolite significantly suppressed microbial activity. Stabilization effects of clay minerals on OM mineralization were measured at compost levels of up to 5%, 10%, and 25% for kaolinite, montmorillonite, and sepiolite, respectively, beyond which the saturation of mineral surfaces occurred.

SEM analysis demonstrated that OM persisted predominantly as particulate organic matter (POM) in the absence of clay minerals, while mineral-associated organic matter (MAOM) was detected in treatments containing clay minerals. Microbial biomass and activity patterns closely aligned with CO₂ emission trends, indicating that clay minerals constrained microbial access to OM depending on clay type and saturation capacity. Rock-Eval® pyrolysis revealed lower hydrogen and oxygen indices in OM incubated with sepiolite and montmorillonite, suggesting enhanced thermal stability. These results were positively correlated with the increased biological stability, as reflected by Q₁₀ values. This study underscores the pivotal role of clay minerals in stabilizing OM, with stabilization efficiencies and saturation thresholds varying significantly among clay mineral types.