Assessments of Organic Carbon Stabilization Using the Spectroscopic Characteristics of Humic Acids Separated from Soils of the Lena River Delta

Abstract

In the Arctic zone, where up to 1024 × 1013 kg of organic matter is stored in permafrost-affected soils, soil organic matter consists of about 50% humic substances. Based on the analysis of the molecular composition of humic acids, we assessed the processes of accumulation of the key structural fragments, their transformations and the stabilization rates of carbon pools in soils in general. The landscape of the Lena River delta is the largest storage of stabilized organic matter in the Arctic. There is active accumulation and deposition of a significant amount of soil organic carbon from terrestrial ecosystems in a permafrost state. Under ongoing climate change, carbon emission fluxes into the atmosphere are estimated to be higher than the sequestration and storing of carbon compounds. Thus, investigation of soil organic matter stabilization mechanisms and rates is quite an urgent topic regarding polar soils. For study of molecular elemental composition, humic acids were separated from the soils of the Lena River delta. Key structural fragments of humic matter were identified and quantified by CP/MAS 13C NMR spectroscopy: carboxyl (–COOR); carbonyl (–C=O); CH3–; CH2–; CH-aliphatic; –C-OR alcohols, esters and carbohydrates; and the phenolic (Ar-OH), quinone (Ar = O) and aromatic (Ar–) groups as benchmark Cryosols of the Lena delta river terrestrial ecosystem. Under the conditions of thermodynamic evolutionary selection, during the change between the dry and wet seasons, up to 41% of aromatic and carboxyl fragments accumulated in humic acids. Data obtained showed that three main groups of carbon played the most important role in soil organic matter stabilization, namely C, H-alkyls ((CH2)n/CH/C and CH3), aromatic compounds (C-C/C-H, C-O) and an OCH group (OCH/OCq). The variations of these carbon species’ content in separated humics, with special reference to soil–permafrost organic profiles’ recalcitrance in the current environment, is discussed.

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

“…A similar content is noted in the mountainous regions of the Crimea [48] with mountain meadow vegetation (43-46% AR). In more northern regions, the content of aromatic structures is significantly lower [27,39,41] than in the studied samples (30-38% AR), this is due to a change in the composition of plants for boreal species (mosses, lichens), which are enriched in lipids (cutins, fatty acids) [29]. In the process of their transformation, predominantly aliphatic compounds are formed.…”

“…HAs are complex systems of high-and low-molecular-weight compounds that are formed during the transformation of plant residues in the soil cover [27]. The use of various methods for analyzing such systems allows us to expand the boundaries in understanding the complex nature of such systems.…”

“…HAs are complex branched chemical systems formed as a result of the decomposition of plant and animal residues in terrestrial and aquatic ecosystems. Various methods are often used for investigation of the HAs of SOM: infrared, ultraviolet and visible spectroscopy, cross polarization, magic angle rotation, molecular fluorescence, and electron paramagnetic resonance spectroscopy [26,27].…”

Assessing Factors Controlling Structural Changes of Humic Acids in Soils Amended with Organic Materials to Improve Soil Functionality

“…Taking T7 and T8 aside, these observations could be attributed to a linear condensation (with age) of secondary amides into more humified aromatic C compounds such as phenols [5,51]. Humified aromatic OM, due to its high content of diverse functional groups, is critical for stabilising soil structure and facilitating the sorption of contaminants and the retention of plant nutrients in soil [13,52,53]. This suggests higher soil quality and better soil functionality in the amended soil from the long-term trials compared with those from the short-and medium-term trials.…”

Soil-geomorphological mapping of Samoylov Island based on UAV imaging

“…The processes of humification of organic matter and the activity of cryogenic processes result in a redistribution of organic matter in soils. We have previously considered the processes of migration and accumulation of soil organic matter in the soils of the Lena River Delta (Zubrzycki et al, 2013;Polyakov and Abakumov, 2021a). Depending on the position in the relief and the activity of cryogenic processes, three types of accumulation can be distinguished.…”