13C NMR and FTIR spectroscopy characterization of humic acids in spodosols under tropical rain forest in southeastern Brazil

The aim of the present study was to characterize the humic acids (HAs) in spodosols developed on quartzitic, sandy, nutrient-poor parent materials under restinga forest. 13C NMR and FTIR spectroscopy were used to obtain information about the mechanism of podzolization in tropical climates in three sandy coastal podzols (Histic Alaquod, Typic Alorthod and Arenic Alorthod) from Cardoso Island (Sao Paulo-Brazil). HAs were extracted from all horizons and 13C NMR and FTIR measurements were performed on all HA samples. Similarities between the surface horizon A from a Typic Alorthod (C14) and the Ho horizon from a Histic Alaquod (H13) were observed. Below the E horizon, the spectra of profile H13 closely resembled the spectra of C14. The increase in alkyl carbon and decrease in acetal carbon, aromatic C and phenolic C with increasing depth suggests enrichment of methylene structures by the microbial biomass. This idea was further supported by an increase in the decomposition of polysaccharides and lignin throughout the profiles. The similarity of the composition may be evidence of vertical translocation throughout the profile, with no incorporation of organic matter (OM) components from other sources such as roots or their residues. The chemical structure of profile Arenic Alorthod (H9) differed from that of the other profiles. The increase in methoxylic, aromatic and phenolic groups as depth increased probably indicates an increase in tannin-like compounds with depth. The higher content of tannins in this soil indicates the presence of more fresh litter and/or a less intense decomposition process than in Histic Alaquod and Typic Alorthod soils. In the latter soil the large amounts of aliphatics and small amounts of lignin-derived products reflect strong aerobic decay. The presence of significant methylene absorption in the FTIR spectra indicates that this alkyl material was dominated by the mobile lignin-derived products, in accordance with the NMR results, which revealed that the HAs have greater aliphatic than aromatic character.