The objective of this study was to investigate the chemical nature of free (FLF) and intra-aggregate (ILF) light fraction of soil organic matter, using infrared spectroscopy as well as to determine whether these fractions are compositionally distinct pools of soil organic matter. Both FLF and ILF were isolated by flotation on NaI solution at a specific gravity of 1.8; for ILF extraction, the soil sample was sonicated after FLF isolation. Samples were collected from two Oxisols from contrasting regions of Brazil (southern and central). Treatments consisted of no-tillage and conventional tillage (ploughing followed by light disc harrowings), under different crop rotation systems. FLF spectra showed features similar to that of plant debris, indicating that these fractions are at early transformation stages. No structural difference was found in the FLF among the different tillage and crop rotation systems. Compared to the FLF spectra, the ILF spectra showed less intense N-H and C-O adsorption bands and were in greater conjugation, which is characteristic of a more humified material. The ILF of soils under conventional tillage presented more changes than the ILF of soils under native vegetation and no tillage. The hydrophobicity (HR) and condensation (CR) indexes, determined by the relationship between the absorption bands of alifatic - CH3, polysacharide C-O and conjugated C=O functional groups, led to the identification of differences in recalcitrance and condensation of the light fractions. Both hydrophobicity and condensation indexes were significantly higher for ILF than for FLF, due to the former's greater moisture degree.
organic matter; density fractionation; cultivated and forest soils; infrared spectroscopy