The humification process increases semiquinone-type free radical concentrations (SFRC) in humus. Their quantification by electron spin resonance spectroscopy (ESR) has been a good indicator of the degree of humification of soil organic matter. As an alternative to relatively complex and expensive ESR spectroscopy, fluorescence spectroscopy was used in this study to evaluate the humification degree of 18 humic acids (HA) extracted from four Brazilian soils under different land use, tillage, or cropping systems. Two fluorescence humification indexes of HA were calculated based on work done by Zsolnay et al. and Kalbitz et al. in 1999, and a third new fluorescence index was proposed. Our proposal is to use the blue wavelength (465 nm) as the HA fluorescence excitation source. As this wavelength is more resonant with the humificated groups present in soil HA samples, our hypothesis is that the resultant fluorescence will provide information about these structures and, therefore, on the degree of humification of the HA. The SFRC varied from 1.90 × 1017 to 14.75 × 1017 spins/g HA, characterizing a wide range of the degree of humification of soil HA. The lowest SFRC occurred in native forests (1.90 × 1017 to 7.50 × 1017 spins/g HA) and pasture soils (2.30 × 1017 to 4.64 × 1017 spins/g HA). In cultivated sites, soil HAs from no-tillage soil were less humified (2.41 × 1017 to 13.30 × 1017 spins/g HA) than those in conventionally tilled soil (5.68 to 14.75 × 1017 spins/g HA). The three fluorescence indexes show the same tendency as SFRC. Our fluorescence method was closely related to procedures found in the works of Zsolnay et al. and Kalbitz et al. (R∼0.9, P < 0.0001) and with SFRC (R∼0.85, P < 0.02), showing their potential as a simple and sensitive technique for evaluating the effect of land use and soil management systems on the humification degree of soil humic acids.
1Embrapa Instrumentação Agropecuária, Rua XV de novembro, 1452, P.O. Box. 741, 13560-970, São Carlos-SP, Brazil.
2Universidade Federal do Rio Grande do Sul, Departamento de Solos, P.O. Box. 776, 90001-970, Porto Alegre-RS, Brazil.
3Universidade de São Paulo, Instituto de Física de São Carlos, P.O. Box. 369, 13560–970, São Carlos -SP, Brazil.
Dr. Milori is corresponding author. E-mail: firstname.lastname@example.org
Received Feb. 27, 2002; accepted July 16, 2002.