The Molecular Composition of Humic Acids in Permafrost Peats in the European Arctic as Paleorecord of the Environmental Conditions of the Holocene

Abstract

The purpose of our research is focused on examination of the transformation regularities of molecular composition of humic acids (HAs) in the hummocky frozen peatlands of the European Arctic as a marker of climatic changes in the Holocene, and assessment of the stabilization of soil organic matter under the conditions of modern climatic warming. Histosols located in the two subzones of the European Arctic served as the research subjects. This territory is actively used for reindeer breeding, which is a vital agricultural branch in the Far North of the Russian Federation. The data obtained reveal the main trends in the formation of HAs from Arctic peatlands under different environmental conditions. Modern peat sediments (top layers) in the middle and late Holocene period formed out of bryophyte residues and contained HAs with long-chain carbohydrate and paraffin structures in their composition. These structures enlarged the dynamic radii of HA molecules, and, thus, caused high average molecular weight values. The more favorable climatic conditions of the early Holocene (the Atlantic optimum) defined the botanical composition of peat, which was dominated by tree and sedge communities with high contents of lignin components and, as a consequence, a larger share of aromatic fragments, characterized by thermo-biodynamic resistance in HAs of horizons in the lower and central profile parts. The molecules of HAs are an archive of paleoclimatic records. The Subboreal and Subatlantic climatic conditions determined the specifics of vegetation precursors and, as a result, the molecular structure of HAs in seasonally thawed layers, with a predominance of long-chain aliphatic fragments. The conversion of HAs from Histosols led to an increase in the proportion of carbon in branched and short-chain paraffinic structures with their subsequent cyclization and aromatization. The results of this process are most clearly manifested in layers formed during the Holocene I and II climatic optima. Higher biologically active temperatures of the seasonally thawed layer of soils at bare spots (without vegetation) determined the accumulation of thermodynamically more stable HA molecules with a high content of aromatic fragments. This contributed to both the stabilization of the SOM and the conservation of peatlands in general.

Relating Paramagnetic Properties to Molecular Parameters of Humic Acids Isolated from Permafrost Peatlands in the European Arctic

“…In areas with poor drainage, sedges and flat-bumped bogs are found. In the Arctic, vegetation cover is affected by the recurring seasonal processes of thawing and freezing of soil water, fluidity of over-watered clay soils, soil frost heaving, crack formation, snow corrasion of the soil surface, turf breaks, and spotting [ 37 , 38 ]. Reindeer grazing and caterpillar transport have a major impact on vegetation cover [ 17 ].…”

“…Data on the composition and physico-chemical properties of peat soils and HA preparations isolated from them are presented in Table 3 . A more detailed characterization and description of the molecular properties of HAs have been published earlier [ 38 , 40 ].…”

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

“…A certain methodological difficulty was the worse (compared to liquid techniques) resolution of solid-phase NMR spectra, especially in polycrystalline samples. Serious progress in the parametric interpretation of such spectra was achieved with the use of special mathematical processing of spectra based on the Fourier series (the so-called Fouriertransformation of spectra), which makes it possible to separate the superimposed singlet signals in the spectrum with a high accuracy and to estimate their integral intensity even with a poor signal-to-noise ratio [28]. In the case of work with heavy nuclei, one often has to deal with the low natural content of isotopes with spin 1/2, such as 29 Si, 15 N and 13 C, which also have a low gyromagnetic ratio as compared with protons, which leads to a lower Zeeman splitting of energy levels and, accordingly, to weak signals.…”

“…A certain methodological difficulty was the worse (compared to liquid techniques) resolution of solid-phase NMR spectra, especially in polycrystalline samples. Serious progress in the parametric interpretation of such spectra was achieved with the use of special mathematical processing of spectra based on the Fourier series (the so-called Fouriertransformation of spectra), which makes it possible to separate the superimposed singlet signals in the spectrum with a high accuracy and to estimate their integral intensity even with a poor signal-to-noise ratio [28].…”

The Role of Organic and Mineral Fertilization in Maintaining Fertility and Productivity of Cryolithozone Soils

“…Application of 7-8 t/ha of manure, (NPK60) for cereals in a typical crop rotation and the saturation of the crop rotation with legumes by 42% allow the maintenance of humus levels at 1.8-1.82%, contributing to the accumulation of HAs of the first and second fractions and expanding the C Has /C FAs ratio to 0.78-0.79. Based on studies on the fruit-bearing crop rotation in Eutric Albic Retisol, the combined application of organic (80 kg/ha for potato) and mineral fertilizers (NPK60) improved the soil agrochemical properties, increased the organic matter quality (C HAs /C FAs rose from 1.10 to 1.78) and productivity (this increased by 6% compared to the grain-grass crop rotation) [92].…”