Soil organic matter is chemically diverse and continuously transforms, presenting a matrix whose composition and reactivity regulate key soil processes. The chemical composition of SOM determines whether compounds are rapidly transformed, persist through mineral association, fuel microbial activity, or participate in nutrient cycling, but these relationships are highly context-dependent and can vary with environmental conditions, land-use, and soil properties. This session brings together research exploring how the chemical complexity of SOM controls its transformation, persistence, and function, bridging molecular-scale processes and soil and ecosystem biogeochemistry. Studies investigating the composition, accessibility, and reactivity of SOM and dissolved organic matter using spectroscopic, isotopic, molecular, and fractionation approaches, as well as research linking SOM chemistry to microbial transformation, mineral–organic interactions, aggregation, nutrient cycling, and carbon stabilization are welcome. We encourage contributions on particulate and mineral-associated organic matter, microbial necromass, mineral surface interactions, organo-mineral associations, and molecular biomarkers that reveal SOM composition and reactivity across scales.