C:N ratio of soil particles drops consistently with increasing particle density across almost all soils studied worldwide to date. The variation in particle density is explained largely by variation in the mass ratio of the organic to mineral phase of these particles, which suggests that the thinner organic coatings may be more N rich.  In fact, considerable empirical data show that proteins and other amino compounds sorb especially stably to mineral surfaces.  To explore mechanisms underlying this trend in C:N ratio, we sequentially fractionated four soils of strongly contrasting mineralogy at densities from 1.65 to 2.90 g/cm³  and analyzed the fractions for measures of organic matter composition.   We also used XRD to document mineralogy of each density fraction. Light fractions (<1.8 g/cm³) were dominantly organic detritus with little mineral content. Mineralogy of the heavier fractions was: 1.8-2.6 g/cm³, layer-silicate clays; 2.6-2.8, quartz, and >2.8 primary minerals.  The Oxisol, which lacked quartz or layer-silicates: all fractions >1.8 g/cm³ contained abundant Fe oxides.  Despite these large mineralogical differences among fractions, C and N decreased with increasing particle density, consistent with published results for other soils.  C/N ratios decreased with increasing density up to about 2.6 g/cm³, then increased.  Δ¹⁴C suggested a consistent increase in C MRT with increasing particle density.  ¹⁵N and ¹³C, however, showed a trend much like C:N. The consistent increase in C MRT with increasing density supports our hypothesis that the thinner, more N-rich coatings are more stable.  The ¹⁵N and ¹³C trends are consistent with increased levels of OM processing by microbes with increasing particle density. That these organic-matter trends occur, despite striking differences in mineralogy among soils and fractions, suggests that protein production by microbes conditions mineral surfaces, thus increasing binding of organics irrespective of the nature of the mineral surface.