Total carbon content and humic substances quality in selected subtypes of Cambisols

Abstract

Cambisols cover an estimated 45% of agricultural soils in the Czech Republic. We aimed our work at stabile forms of organic carbon and humic substances quality in Cambisols under different types of soil management (grassland and arable soil). Object of our study were the following subtypes of Cambisols: Eutric Cambisol (locality Vatín – arable soil), Eutric Cambisol (locality Vatín – grassland), Haplic Cambisol (locality Náměšť n/Oslavou – arable soil), Leptic Cambisol (locality Ocmanice – grassland), Haplic Cambisol (locality Nové Město na Moravě – arable soil), Haplic Cambisol (locality Přemyslov – Tři Kameny – grassland), Arenic Cambisol (locality Pocoucov – arable soil), Dystric Cambisol (locality Sněžné – arable soil), Dystric Cambisol (locality Velká Skrovnice – arable soil), Dystric Cambisol (locality Vojnův Městec – arable soil). Non-destructive spectroscopic methods such as UV-VIS spectroscopy, synchronous fluorescence spectroscopy (SFS) and 13C NMR spectroscopy for humic substances (HS) quality assessment were used. Total organic carbon (TOC) content was determined by oxidimetric titration. Fractionation of HS was made by short fractionation method. Isolation of pure humic ­acids (HA) preparation was made according to the standard IHSS method.Results showed that TOC and humus content varied from 2.70 % (grassland) to 1.3 % (arable soil). Ave­ra­ge HS sum was 8.4 mg / kg in grassland and 6.4 mg / kg in arable soil. Average HA sum was 3.6 mg / kg in grassland and 3 mg / kg in arable soil. Fulvic acids (FA) content was 4.7 mg / kg in grassland and 3.7 mg / kg in arable soil. HS quality was low and very similar for all studied samples. HA/FA ratio low (< 1). HS absorbance in UV-VIS spectral range was low and similar in all studied samples. Higher absorption in this spectral range was closely connected with higher HS content. Also in 2D-synchronous fluorescence scan spectra similar shape of spectral lines was detected. Relative fluorescence intensity strongly depended on HA and FA content. Five main fluorescence peaks for HA and FA presence at λex./ λem.: 468/488, 482/502, 450/470, 376/396, 340/360 at constant difference of ∆λ=20 nm were measured. 13C NMR spectra of HA isolated from Eutric Cambisol showed low aromaticity degree and high content of aliphatic moieties. HA were young (= less aromatic compounds in HA molecule) and no differences in HA structure were detected to compare arable soil and grassland.

Keywords

• humus fractionation
• UV-VIS
• SFS and ¹³CNMR spectroscopy
• Cambisols