Spectroscopic characteristics of humates isolated from different soils

Abstract

  Spectral characterisation of soil humic substances is one of the important methods for their quality identification. In this work, two optical methods (UV-VIS and SFS) were used. The absorbance in the spectral range of 300–700 nm was measured using spectrometer Varian Cary 50 Probe. Fluorescence (SFS) in the range of 255–655 nm was performed by spectrofluorimeter Aminco Bowman. Five Czech soil humates samples (Leptic Cambisol, Haplic Cambisol, Eutric Cambisol 1-arable soil, Eutric Cambisol 2-grassland, Haplic Chernozem) were compared. The basic soil properties were determined by the commonly used methods. Colour indexes (Q<sub>4/6</sub>) were calculated from the absorbance of humic substances in UV- VIS spectral range. Fluorescence indexes (F) were calculated from SFS spectra at Δλ = 55 nm (as a ratio RFI<sub>468</sub>/RFI<sub>522</sub>). Also, the classical method of humic substances fractionation to assess their quality was applied. The comparison is given of the calculated parameters from different spectral regions and humic substances fractionation. The results showed linear correlation between carbon content (C weigth %) and fluorescence indexes (R<sup>2 </sup>= 0.91), between total organic carbon content in soil and fluorescence indexes (R<sup>2 </sup>= 0.92), as well as between colour indexes (Q<sub>4/6</sub>) and humic substances content. The same main fluorophores at the wavelengths λ<sub>ex</sub>/λ<sub>em</sub>= 467/522 nm were observed in all soil humates samples.