Humus formation in soils of the Lena River Delta

Abstract

The aim of the study. Nowadays close attention is paid to polar soils due to the expected landscape transformation rate under the predicted climate crisis. Intensive degradation of permafrost and the release of nutrients from their frozen state can lead to an increase in the emission of greenhouse gases into the atmosphere, as well as the loss of landscapes. The aim of the study was to investigate the peculiarities of organic residues formation and humification degree as well as humus structure and functioning in soil the Lena River Delta.

Location and time of the study. The study was conducted on the Samoylov and Sardach Islands in the Lena River Delta (Yakutia, Russia). Field studies were performed during the summer of 2019.

Objects and methodology. Soils of the Lena River Delta from the Samoylov Island (flooded area) and Sardakh (non-flooded zone), i.e. Subaquatic Fluvisol (Arenic) and Histic Cryosol (Siltic) were the objects of the study, respectively. To examine the features of humification chemical-analytical, sedimentation, micromorphological methods were used, as well as CP/MAS ¹³C-NMR spectroscopy.

Main results. The data obtained indicate a high diversity of soils and soil formation conditions in the Lena River Delta. Under non-flooded conditions Histic Cryosol (Siltic) were formed in the flooded parts of the Samoylov Island, Subaquatic Fluvisol (Arenic) were formed. These soils play an important role in the global carbon cycle, accumulation, transformation and deposition of condensed high- and low molecular mass organic compounds in the composition of soils and permafrost. The main soil micromorphology features were identified. In the young landscapes the soil microstructure was represented by poorly sorted sand with a circular type of optical orientation of the soil plasma (which indicated the influence of the river), as well as vertically oriented micas (muscovite/biotite). Soils influenced by the floodplain process were characterized by the presence of coarse amorphous humus. Due to the long-term effect of the freezing/thawing processes on the permafrost-affected soils the biogenic aggregates were formed. In such organo-mineral microaggregates humus is fixed in the composition of mineral components consisting of particles of quartz, micas and Mn-Fe nodules and is in a stable state (physical stabilization of humus). To analyze the molecular composition the ¹³C NMR spectroscopy method was used. According to ¹³C NMR spectroscopy data, up to 37% of aromatic structural fragments accumulated in soils, which indicates the process of condensation of molecules in humic acids, thus showing a relatively high level of humus stabilization in the soils of the Lena River delta. From the data obtained, three main groups of chemical structural fragments that accumulate in the delta soils can be distinguished, such as C,H - alkyls ((CH₂)n/CH/C and CH₃), aromatic compounds (C–C/C–H, C–O) and the OCH group (OCH/OCq). In the studied humic preparations aliphatic structural fragments of HAs (63–64%) predominated, which indicated a deficiency of lignin and lignin-like compounds in the composition of humification precursors. The predominance of aliphatic structures is typical of humic substances formed under reduction conditions, including the aqueous humic substances, as well as in soils, the precursors of humification of which are typical tundra vegetation (mosses and lichens) with a predominance of carbohydrates (up to 80%). A significant amount of aromatic fragments accumulated in the delta in the alluvial soils of the first terrace of the river (flooded zone). This is apparently due to the formation of vascular plants in the flooded areas of the Lena River Delta. The chemical composition of vascular plants includes components such as tannins, flavonoids and lignin (arenas). Thus in the samples formed under vascular plants with a lignin content of about 30%, an increase in signals in the range of 110-160 ppm were observed. Aromatic and carboxyl fragments in the structure of HA were formed during the transformation of lignin, which leads to the resistance of HAs to biodegradation.

Conclusion.  Under the floodplain condition humus is represented by coarse amorphous forms, whereas without the influence of flooding humus is fixed in mineral microaggregates. Specific humic acids composition in the studied regions is determined by bioclimatic conditions, cryogenic processes and the composition of humification precursors. High abundance of aromatic structures in humus substances suggests relatively stable humus status (most likely due to the putative condensation of humic acids) in the Lena River Delta during the study period.