EPR Spectroscopy as a Tool to Characterize the Maturity Degree of Humic Acids

Abstract

The major indicator of soil fertility and productivity are humic acids (HAs) arising from decomposition of organic matter. The structure and properties of HAs depend, among others climate factors, on soil and anthropogenic factors, i.e., methods of soil management. The purpose of the research undertaken in this paper is to study humic acids resulting from the decomposition of crop residues of wheat (Triticum aestivum L.) and plant material of thuja (Thuja plicata D.Don.ex. Lamb) using electron paramagnetic resonance (EPR) spectroscopy. In the present paper, we report EPR studies carried out on two types of HAs extracted from forest soil and incubated samples of plant material (mixture of wheat straw and roots), both without soil and mixed with soil. EPR signals obtained from these samples were subjected to numerical analysis, which showed that the EPR spectra of each sample could be deconvoluted into Lorentzian and Gaussian components. It can be shown that the origin of HAs has a significant impact on the parameters of their EPR spectra. The parameters of EPR spectra of humic acids depend strongly on their origin. The HA samples isolated from forest soils are characterized by higher spin concentration and lower peak-to-peak width of EPR spectra in comparison to those of HAs incubated from plant material.

Metal-Free Organic Radical Spin Source

“…This is in contrast to most neutral π radicals, which are insulators or poorly conducting semiconductors with σ < 10 –10 S cm –1 . Furthermore, using the doubly integrated ESR spectra of the EBR calibrated against the standard DPPH S = 1/2 radical, we obtain the spin density in the EBR film and, consequently, a mean distance between the radical spin centers of ∼0.4 nm, for which a strong exchange interaction is expected. , This is consistent with the ESR spectrum having a single Lorentzian shape, a feature characteristic of high spin density radical systems with strong exchange between neighboring molecules. , Therefore, a contribution to spin transport within the radical film from both mechanisms cannot be ruled out.…”

“…30,31 This is consistent with the ESR spectrum having a single Lorentzian shape, a feature characteristic of high spin density radical systems with strong exchange between neighboring mole- cules. 52,53 Therefore, a contribution to spin transport within the radical film from both mechanisms cannot be ruled out. A contribution to the observed increase of ΔH pp can also arise from factors unrelated to spin pumping, for example, due to a permanent alteration of the nature of the radical molecule itself when deposited on a metallic surface, resulting in the increase of ΔH pp .…”

“…cules. 52,53 Therefore, a contribution to spin transport within the radical film from both mechanisms cannot be ruled out. A contribution to the observed increase of ΔH pp can also arise from factors unrelated to spin pumping, for example, due to a permanent alteration of the nature of the radical molecule itself when deposited on a metallic surface, resulting in the increase of ΔH pp .…”

The Use of Spectroscopic Methods to Study Organic Matter in Virgin and Arable Soils: A Scoping Review

“…This circumstance accounts for both the unusual stability of the paramagnetism of HSs (unpaired electrons do not recombine in the liquid phase) and the sensitivity of EPR spectra to the chemical structure and size of highly dispersed particles. There is also a fairly widespread belief (in half of the papers where authors discussed the nature of SOM paramagnetism) that the paramagnetism of HSs is mainly due to semiquinone radicals and is justified by the absence of significant differences in the shape of the EPR spectra of the original samples and those treated with alkaline solutions, as well as the positive correlation between the signal intensity and the content of quinoid groups [101,102]. The formation of FRs during enzymatic oxidation of HSs containing phenolic fragments has been noted by many researchers.…”

Characteristics of Humic Acids in Drained Floodplain Soils in Temperate Climates: A Spectroscopic Study

“…More humified organic matter is characterized by an increase in the aromaticity of the molecules, an increase in carbon and decrease in hydrogen content, as well as an increase in the content of carboxyl and methoxy groups compared to less humified matter [18]. Advanced instrumental techniques such as FTIR, 13 C CP MAS NMR/ 1 H NMR, or EPR spectroscopy can help in describing the detailed chemical properties of humic substances as well as following the direction of humification [19][20][21]. Decreasing in SOM, particularly HAs (a very stable humus fraction), contributes to an increase in compactness of the soil, diminishing soil aeration, and the appearance of reductive chemical conditions [22], as well as reducing the stability of soil aggregates.…”