Ongoing climate change is making the large pool of organic matter (OM) stored in Arctic permafrost vulnerable to mobilization; thus, garnering a deeper understanding of molecular transformations within the abundant pool of soil OM, specifically humic substances, is crucial. Here we present the first high-resolution mass-spectrometry examination of molecular compositions of humic acid (HA) and fulvic acid (FA) isolated from organic-rich deep yedoma (Pleistocene age ice-rich permafrost) and alas (thermokarst deposit formed during permafrost thaw) cores. The FA fractions were dominated by oxygen-rich unsaturated compounds, whereas the HA fractions were mostly composed of relatively reduced saturated and aromatic moieties. A substantial increase in contribution of both CHO-only and N-containing aliphatic compounds was observed in the HA fractions of the yedoma OM with depth, whereas the alas HA fractions were depleted in aliphatics but enriched with condensed and hydrolyzable tannins. The observed differences in compositional space of the immobile OM stored in the deep yedoma versus alas deposits were consistent with evolution of OM during thermokarst lake genesis, implying intense microbial degradation of N-rich OM released from the yedoma deposits and accumulation of highly degraded, plant-derived OM. The patterns of molecular transformations of OM were apparent in compositional space of the least degraded HA fractions as compared to much more oxidized FA fractions. This shows great promise of molecular exploration of the alkali-extracted OM, comprising up to 50% of the total organic carbon in deep permafrost both for paleoreconstructions and predictions of climate feedback to released OM due to permafrost thaw.
The paper is devoted to the development and performance testing of a soil conditioner based on leonardite humic substances (LHS) modified with 3-aminopropyltriethoxysilane (APTES). The modified HS were obtained by adding APTES to LHS solution at different mass ratios of LHS and APTES, followed by the investigation of siloxane structures using 31Si NMR spectroscopy. The Urbic Technosol was used as a model soil. The size and amount of water-stable soil aggregates were estimated using wet sieving and laser diffraction, respectively. Toxicity was evaluated by monitoring microbial substrate-induced respiration (SIR) and seedling bioassay. Laboratory column experiments demonstrated an increase in water-stability of the 3–5 mm soil aggregates after LHS-APTES application. Field tests showed an increase in the average weighted diameter of micro aggregates (from 59 to 73 μm) and water-stable macroaggregates (from 1.6 to 2.9 mm) due to the LHS-APTES amendment. A substantial increase in SIR from 5 to 9 mg CO2 (kg h)−1 was detected. Better survival of seedlings was observed. The obtained beneficial results indicate that APTES-modified HS can be successfully used as a soil conditioner. The formation of extended siloxane networks was suggested as the main mechanism of the observed improvement in the structure of the amended soils.
From the soddy podzolic soils and typical chernozems of different texture and land use, dry 3-1 mm aggregates were isolated and sieved in water. As a result, water stable aggregates and water unstable particles composing dry 3-1 mm aggregates were obtained. These preparations were ground, and contact angles of wet ting were determined by the static sessile drop method. The angles varied from 11° to 85°. In most cases, the values of the angles for the water stable aggregates significantly exceeded those for the water unstable compo nents. In terms of carbon content in structural units, there was no correlation between these parameters. When analyzing the soil varieties separately, the significant positive correlation between the carbon content and contact angle of aggregates was revealed only for the loamy clayey typical chernozem. Based on the multivariate analysis of variance, the value of contact wetting angle was shown to be determined by the structural units belonging to water stable or water unstable components of macroaggregates and by the land use type. In addition, along with these parameters, the texture has an indirect effect.
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