It is known from the literature that the rheological behavior of soils is largely dependent on the water content in pastes and soil organic matter forming the basis of organomineral soil gels. With an increase in soil moisture, gels can swell. As a result, the viscosity of the soil paste should change. The objective of this study was to assess the effect of soil moisture on the viscosity of soil paste. Arable soil horizons were used in this work: sod-podzolic, gray forest, leached chernozem, and chestnut. During the experiments, the soil moisture was changed, whereas the water content in the pastes in each soil type remained unchanged. The viscosity of the soil paste was determined by vibration viscometry, and the size of organomineral particles in pastes was determined by laser diffractometry. Two paste viscosity peaks depending on the soil moisture were obtained for all samples studied. The paste viscosity peaks were explained from the perspective of changes in the structure of humic substances in organomineral gels upon reaching critical concentrations: micelles-supramolecular formations-fractal clusters. Apparently, the transition between structural forms of humic substances under mechanical action on pastes is accompanied by the disintegration of large gel particles and the formation of a more balanced form of humic substances at a given water content.

Water scarcity is a critical problem around the world, and superabsorbent hydrogels has attracted growing attention in water management for handling water deficiency during agricultural and forestry practices. Herein, intending to apply gelatin hydrogel as soil conditioner, humic substances (HS) extracted from Chinese medicine residue compost were used to modify gelatin hydrogel through either physical mixing or chemical cross-linking. The results demonstrated that low level of HS could improve the hardness and rheological properties of the hydrogels, however, the gel strength significantly decreased when the concentration of HS rose up to 16 g/L. As revealed by TEM and XRD, chemical cross-linking reaction promoted the development of denser network structures, thereby improving the hardness and rheological properties of the hydrogels. Subsequently, applying HS at a concentration of 3 g/L was found to be preferable for enhancing the swelling ratio of the gelatin hydrogels, and lightweight substrates amended with the resultant hydrogels displayed superior water retention ratio (17.23 +/- 0.79 % for GelHS3 and 17.74 +/- 1.31 % for GelHS3-EDC). Furthermore, it was proved that HSincorporated hydrogels can effectively keep moisture for the growth of Melaleuca alternifolia (Maiden & Betche) Cheel saplings under drought stress. These findings suggest that humic substances can be utilized to modify hydrogels for use as soil conditioners.

Climate change results in increased attention to factors determining the dynamics of biogeochemical processes in the active layer of biosphere (seasonally thawing/freezing soils) at the boundary with permafrost. There are geological, geomorphic, and weather conditions associated with atmospheric precipitation, which determine the differences in the segregation and the thawing depth of permafrost in various regions. The research area is the Bureya River Basin with discontinuous permafrost within unique landscape units: Mari. They are formed in swampy and poorly drained areas where the active soil layer is in contact with permafrost, and they exert a direct impact on the quality of surface waters. The results of the study include data on the chemical composition of water in tributaries of different orders; spectral characteristics of soluble organic matter (OM) in water extracts (WEs) of soils from different horizons of the active layer; and the assessment of the activity of microbial complexes in relation to humic substances (HSs), depending on the permafrost depth. The heterogeneity of landscapes is an important factor, influencing the transport of OM into watercourses. Microbial complexes of the active layer involved in the transformation of HSs play a decisive role in changing the composition of soluble OM. It is experimentally shown that temperature affects the qualitative composition of metabolism products of HSs, including the ratio of aliphatic and aromatic fragments to chromophoric groups responsible for the color of natural waters.