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The associations between organic and mineral matter in the horizons of an Amazonian humiluvic Spodosol profile were studied by investigating the composition of the constituent organic compounds, the surface wettability and the relationships among the chemical, structural, and microstructural properties using Fourier Transform Infrared (FTIR) spectroscopy and contact angle (CA) analysis. With increasing the profile depth, the C[sbnd]H FTIR absorption decreased accompanied by a relative increase in C[dbnd]O FTIR absorption. A number of significant correlations were found between the experimental variables measured. In particular, the positive correlation between soil organic carbon (SOC) (1.43 to 31.13 g kg) and intensity of aromatic structural C[dbnd]C peak at 1620 cm downward the soil profile suggested the existence of higher amounts of more stable organic compounds with depth. The negative correlation between the intensity of aliphatic structures peaks from 3000 to 2800 cm and the clay minerals content indicated an increasing soil wettability with depth. The relative affinity of organic matter to mineral surfaces changed as a function of the mineral type distribution along the soil profile. As well, the chemical composition of organic matter and the type of phase fractions influenced whole soil wettability based on the SOC/clay relation to CA.
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