This study was carried out to characterize the possible changes in the chemical nature of humic acids extracted from an Oxisol (0-10 cm) by Fourier transform infrared spectroscopy (FTIR), in the experimental area of Embrapa Meio Ambiente, in Jaguariúna (SP), Brazil. The data were obtained after six corn cultivations, and application of variable doses of sewage sludge (dry basis) from Barueri (BSS), in the treatments: BSS0 - control without sewage sludge application; NPK - mineral fertilization as required by corn; application of BSS to supply one (BSS1), two (BSS2), for four (BSS4), and eight (BSS8) times the N required by corn. As a reference, a forest area close to the experiment was also sampled. FTIR spectra of humic acids (HA) and the sewage sludge used in the experiments were obtained, and the hydrophobicity (HI) and condensation (CI) indexes for the HA samples were calculated. Small changes in the chemical nature of HA from soil treated with BSS were observed in comparison to that from the control soil. The HA analyzed contained aromatic, phenolics, and carboxylic groups, aliphatic structures, carbohydrates and mineral impurities. The continuous use of high BSS doses reduced the aliphatic-C and condensation degree of HA in the soil, which indicates the presence of organic materials with higher bioavailability in these areas. In the area under forest, the HA fraction was characterized by the hydrophobic character and low condensation degree.
organic matter; bioavailability; hydrophobicity index; humic substances
