Characterization of Organic-Matter in Particle-Size Fractions of Vertisols

The nature of organic matter (OM) of five Vertisols was investigated by means of Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFT) on whole soil samples and on stable aggregates of different size, separated by water dispersion. This method allows the IR spectra of soil OM to be obtained in its original state, unaltered by chemical treatments and with part of the functional groups bound to the mineral components. Further information on the nature of OM was obtained by electrofocusing (EF) analyses of the 0.1 M Na4P2O7-NaOH extracts of soils and of water stable soil aggregates. The relationship of OM with soil fabric was investigated by determining the organic carbon distribution in the different water-stable fractions. Our results show that OM is concentrated in the silt size aggregates and its nature appears rather uniform in the five Vertisols. In particular, DRIFT analyses reveal the existence of very strong hydrogen bonds between the OM and the soil mineral fraction and the presence, in the natural soils, of compounds, such as cyclic anhydrides. Moreover, aromatic substances appear concentrated in the water-stable, silt-size aggregates, while relatively simple aliphatic compounds seem to prevail in clay fractions. In the "apparent" silt fraction EF analyses show the presence of considerable amounts of organo-metal complexes, probably responsible for the aggregation of fine clay particles.