Organic matter study of whole soil samples using laser-induced fluorescence spectroscopy

Fluorescence spectroscopy relies on the fluorescence emitted by rigid conjugated systems and thus can be used to assess the soil organic matter (SOM) humification. This technique is generally applied to solution samples of humic substances, and so far no information exists about its applicability to whole untreated soil samples. The laser-induced fluorescence (LIF) spectroscopy is proposed as a novel technique to assess the organic matter humification in whole soil samples. We sampled the 0- to 2.5-, 2.5- to 5-, 5- to 10-, 10- to 15-, and 15- to 20-cm layers of three Oxisols of long-term experiments located in two sites of the Brazilian Cerrado. The humification index based on LIF spectroscopy (H(LIF)) of whole soil samples showed a close correlation with the humification indexes A4/A1, I465/I399, and A465 obtained after fluorescence spectroscopy analysis of the dissolved humic acids. The H(LIF) in soils under native cerrado or subjected to no-tillage increased from the top to the deepest layer, which is consistent with the deposition of labile organic matter from plant residues on the soil surface. The soils subjected to conventional tillage, however, showed relatively constant H(LIF) along the profile, possibly because homogenization imparted by disturbance of the arable layer. Accordingly, for the two top layers, the soils under no-tillage showed a lower H(LIF) than for conventionally tilled soils. Laser-induced fluorescence spectroscopy is a promising technique to assess humification in whole soil samples, particularly in Oxisols, which due to high concentration of Fe(3+) are not feasible to electron spin resonance (ESR) and Carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy, unless previous treatment is employed.