Humification degree of soil humic acids determined by fluorescence spectroscopy

The humification process increases semiquitione-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 X 10(17) to 14.75 X 10(17) spins/g HA, characterizing a wide range of the degree of humification of soil HA. The lowest SFRC occurred in native forests (1.90 X 10(17) to 7.50 X 10(17) spins/g HA) and pasture soils (2.30 X 10(17) to 4.64 X 10(17) spins/g HA). In cultivated sites, soil HAs from no-tillage soil were less humified (2.41 X 10(17) to 13.30 X 10(17) spins/g HA) than those in conventionally tilled soil (5.68 to 14.75 X 10(17) 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. (Rsimilar to0.9, P < 0.0001) and with SFRC (Rsimilar to0.85, 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.