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The effects of humidity and metal ions in the determination of humic organic free radicals concentration (HOFRC) were studied by electron paramagnetic resonance (EPR). Increasing the sample humidity caused a decrease in both HOFRC, probably due to free radicals recombination, and in the derivative peak-to-peak line width (ΔH), probably due to a higher spin–spin relaxation time (T2) as the molecular mobility increased (shorter correlation time). Diamagnetic ions (Na+, Zn2+ and Al3+) did not affect the HOFRC, while paramagnetic ions (Fe3+, Mn2+ and Cu2+) induced an intense HOFRC decrease. This effect was associated with dipolar-relaxation enhancement at high (above 0.01%) paramagnetic ion concentrations, since the inhomogeneity parameter decreased to less than 1. Further, a possible exchange interaction and/or shift in the free radicals equilibrium reactions might have occurred at low paramagnetic ion concentrations (below 0.01%). [Copyright &y& Elsevier]

Long-term (5- and 9-year) effects were evaluated of two tillage regimes (conventional tillage: CT; and no-tillage: NT) and two cropping systems (oat/maize: O/M; and oat+vetch/maize+cowpea: O+V/M+C) on characteristics of humic acids (HAs) from surface layer (0 to 25 mm) of a subtropical Brazilian Paleudult soil. Generally, soil HA samples fromconservation management systems with no soil disturbance (NT) and high crop residue addition (O+V/M+C) showed lowest humification degree,as demonstrated by lowest concentration of semiquinone-type free radicals, determined by electron spin resonance (ESR), and lowest total fluorescence (TF), which is proportional to area under fluorescence spectrum; however, cropping systems had less effect than tillage regimes on two spectroscopic parameters. Since all HA samples presented a maximum fluorescence emission peak around 518 nm, when excited with blue light, we believe that differences in the fluorescence intensity could be attributed to concentration of similar condensed aromatic moiety. This assumption was consistent with strong correlation with stable semiquinone-type free radicals (r=0.84, P<0.01), which are believed to be stabilized in humic substances by condensed aromatic structures. The results showed positive effects of conservation management systems on soil humus characteristics, which may therefore be used as alternative parameters in evaluating management system quality. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]