Some biomolecules in soil organic matter (SOM) are intrinsically more resistant to microbial decomposition than are other SOM components. Their resistance can be altered by soil properties and by land management, which can affect the formation and stability of soil organic matter and in turn soil processes.  Selected examples follow. First, black carbon in three Midwestern soils was found to be preferentially concentrated in the coarse clay subfraction.  It consisted principally of aromatic C that might had originally been charcoal formed during prairie fires and that had since attracted aliphatic biogenic C.  The black C had old radiocarbon ages and was interpreted to be less bioavailable than more recent biogenic compounds.  Medium and fine clay subfractions were dominated by modern biogenic C, which was dominantly aliphatic.  Second, in a study of two Iowa soils, humic acid in the clay fraction was enriched in readily oxidizable molecules compared to the humic acid in the silt fraction. Humic acid in a poorly drained clayey soil had more aromatic C, including charcoal–like moieties, while humic acid in a sandy, well drained soil had more aliphatic compounds, readily combustible carbohydrates and lignin. Third, lignin-derived phenolic compounds often accumulate in flooded soils, apparently contributing simultaneously to both C sequestration and an impedance of N cycling.  Their accumulation is affected by crop rotation and also soil aeration during crop residue decomposition.  In aerobic soils, phenols have been associated with medium-term soil aggregation. Improved chromatographic and spectroscopic analyses for phenols will better elucidate their origins and their roles in C stabilization and nutrient cycling.   Finally, a few studies that characterized SOM chemistry following long-term animal manure incorporation showed mainly an accumulation of fatty acids. These examples reflect the interplay of soil properties and land management that affects the resistance of biomolecules to microbial decomposition and their enrichment in soils.