Elemental and Molecular Composition of Humic Acids Isolated from Soils of Tallgrass Temperate Rainforests (Chernevaya taiga) by 1H-13C HECTCOR NMR Spectroscopy

Abstract

The soils of Chernevaya taiga (tallgrass fir-aspen hemiboreal rainforest) have high fertility in comparison with oligotrophic analogs formed in boreal taiga. We have studied humic acids isolated from the soils of Chernevaya and oligotrophic taiga in the Novosibirsk, Tomsk, Kemerovo and the Altai regions of Russia and for the first time the structural and molecular composition of humic acids was determined using 13C CP/MAS and 1H-13C HETCOR NMR spectroscopy. According to data obtained in this study, up to 48% of aromatic compounds accumulate in the soils of Chernevaya taiga, which is higher than in the oligotrophic taiga and comparable with this rate of steppe Chernozems. In the course of active processes of transformation of organic matter, a significant number of aromatic fragments accumulates in the middle horizons of soil profiles. Using 13C CP/MAS spectroscopy, it was possible to identify the main structural fragments (aliphatic and aromatic) that formed in humic acids of the Chernevaya taiga. The HETCOR experiment made it possible to accurately determine the boundaries of chemical shifts of the main groups of structural fragments of humic acids. Our results demonstrate that the stabilization of organic compounds occurs in the soil of the Chernevaya taiga, which leads to the resistance of organic matter to biodegradation that is not typical for benchmark soils of boreal environments.

Distribution of Molecular Weight of Humic Substances Isolated from Soils of Tallgrass Temperate Rainforests (Chernevaya Taiga)

“…N = 9. r cr = 0.59. The data of 13 C NMR spectroscopy analysis based on previously obtained data [26]. Pearson's correlation was used to analyze the correlation between the MW distribution and the content of structural fragments by 13 C NMR spectroscopy (Table 3).…”

“…The main part of Chernevaya taiga is located at altitude ranging from approximately 200 to 700/800 m on the western, windward macroslopes of the mountains and foothills of southern Western Siberia (Figure 1). In a previous study [26], we used 13 C and 1 H- 13 C NMR spectroscopy to investigate the composition of HAs. We discovered that Chernevaya taiga soils accumulate up to 48% of aromatic structural fragments in the middle of the soil profile, which differs significantly from soils from other boreal forests, indicating a high degree of aromatic compounds in tall-grass forest plant restudies [27][28][29].…”

“…Fragments by 13 C NMR Spectroscopy A characteristic feature of the HAs isolated from the soils of Chernevaya taiga is an increase in both aromaticity [26] and the content of the low-MW fraction with depth. This may be due to more active processes in the transformation of organic matter in the underlying horizons.…”

“…In a previous study [26], we used 13 C and 1 H- 13 C NMR spectroscopy to investigate the composition of HAs. We discovered that Chernevaya taiga soils accumulate up to 48% of aromatic structural fragments in the middle of the soil profile, which differs significantly from soils from other boreal forests, indicating a high degree of aromatic compounds in tall-grass forest plant restudies [27][28][29].…”

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

“…Resonances from functionalized carbon chains (substituted with O-or other hetero-element-containing groups), as well as carbohydrate resonances (δ C /δ H : 35-110/2.5-5.5 ppm), are present in the oxygencontaining aliphatic region. Aromatic functional groups resonate in the weak field (δ C /δ H : 110-150/6.0-8.0 ppm) [38,39]. The hydroxyalkyl and alkyl protons are well identified, while there are significant difficulties in identifying the protons of aromatic fragments [34].…”