Organic matter plays a major role in the chemical and physical attributes of highly weathered soils usually found in tropical and subtropical regions. In the present work, soil organic matter (SOM) of six representative Brazilian Ferralsol profiles was examined by chemical analyses, solid state 13 C and 15 N nuclear magnetic resonance (NMR) spectroscopy and electron spin resonance (ESR) spectroscopy. The HF treatment, realized with the soil samples prior to the spectroscopic analysis, promoted both C and N enrichment, and no selective dissolution of specific C-compounds was identified. The recovery of C and N ranged between 47% and 66% in the A horizon and decreased pronouncedly with depth. The solubility of SOM after HF treatment, especially in deeper horizons, was related to its occurrence in organo-mineral associates. The results obtained by 13 C NMR revealed that the SOM of A horizons of the Brazilian Ferralsols is characterized by a considerable amount of O-alkyl (29-38%), followed by lower proportions of alkyl (24-30%) and aromatic structures (10-14%). The organic nitrogen occurred mainly as amide, as indicated by the 15 N NMR analysis. A similar pattern was exhibited by the SOM of lower horizons. The C/N ratio varied between 8 and 21 and tended to decrease with depth. The same trend was found for the semiquinone-type free radicals concentrations, determined by ESR spectroscopy. The occurrence of a SOM showing a medium decomposition degree containing structures of low chemical recalcitrance points to a stabilization of organic matter mainly through surface interactions with iron oxides and kaolinite. D

Very intense visible green photoluminescence ͑PL͒ was observed at room temperature in structurally ordered-disordered BaZrO 3 powders. Ab initio calculations, ultraviolet-visible absorption spectroscopy, electron paramagnetic resonance, and PL were performed. Theoretical and experimental results showed that local defects in the cubic structure caused byplay an important role in the formation of hole-electron pairs, giving rise to a charge gradient in the structure which is responsible for PL emission.

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