Adsorption of Fulvic Acid and Water Extractable Soil Organic Matter on Kaolinite and Muscovite

The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere. The adsorption of humic acids on minerals is accompanied by their fractionation in size, composition, and amphiphilicity, thus decreasing their heterogeneity. Despite a strong interest in studying the regularities and mechanisms of the interaction between natural organic matter and layered aluminosilicates, it is necessary to take into account the natural diversity of soil organic matter, adsorption conditions, and mineral composition. This study was designed to investigate the adsorption regularities of fulvic acid (FA) and water-extractable organic matter (WEOM) isolated from horizon H of peaty-podzolic-gleyic soil on kaolinite and muscovite. Sorbates and sorbents were examined by the following methods: high-pressure size exclusion chromatography (HPSEC), high-performance liquid chromatography (HPLC), and potentiometric titration. The specific surface areas of the sorbents were determined by the sorption of N2 molecules. We found that hydrophobic components of FA and WEOM are mainly adsorbed on mineral surfaces. The adsorption of FA and WEOM on kaolinite and muscovite is followed by decreased hydrophobicity of organic matter and decreased heterogeneity of its amphiphilic properties in an equilibrium solution. At pH levels around 6, sorption of organic matter from FA solution containing 19% and 81% hydrophilic and hydrophobic components, respectively, onto kaolinite and muscovite occurs mainly due to hydrophobic components. Hydrophobic interactions on siloxane surfaces are the main mechanism to fix FA on both minerals. Kaolinite adsorbs slightly more organic carbon per unit area than muscovite. The adsorption of WEOM from a solution with 41% hydrophilic and 59% hydrophobic components results not only from hydrophobic and hydrophilic components but also from hydrophobic and electrostatic interactions and depends on pH. The most hydrophobic fractions of organic matter are adsorbed from the hydrophobic components on the surface of both minerals. Under conditions of the performed experiments at pH < 5, more WEOM is adsorbed on muscovite than on kaolinite.

Quantitative and Qualitative Responses of Soil Water-Extractable Organic Matter to Carbon and Nitrogen Management Practices in Loess Soil

Soil-dissolved organic matter (DOM) drives the carbon (C) and nitrogen (N) cycles in agroecosystems. Despite many studies on DOM dynamics, hardly any attention has been directed toward DOM quality, particularly DOM composition. The aim of this study was to elucidate how C and N management practices alter soil water-extractable organic matter (WEOM) in a loess soil agroecosystem. Field experiments were conducted with a winter wheat monoculture. Three N fertilization rates (0, 120, and 240 kg ha−1 year−1) were applied for 17 years (2002–2019), combined with five C practices (zero, low, and high rates of sheep manure or wheat straw) for three years (2016–2019). The results reveal that soil organic carbon (SOC) and water-extractable organic carbon (WEOC) concentrations in the topsoil (0–20 cm) were increased by organic amendments considerably but were not affected by N fertilization. The fluorescence excitation–emission matrix spectra (EEM) of WEOM were resolved to two humic-like components (C1 and C2) and two soluble microbial byproduct-like components (C3 and C4). The proportions of C1 and C2 were increased, while the proportion of C3 was decreased by both C and N management practices. In conclusion, organic amendments increased both WEOM quality and its proportion of humic-like components, whereas N fertilization increased the proportion of humic-like components without variations of WEOM quality in the topsoil of loess soil.

Trichoderma-Enriched Vermicompost Extracts Reduces Nematode Biotic Stress in Tomato and Bell Pepper Crops

Root-knot nematode (RKN) is a serious threat to crops worldwide due to the difficulty in controlling it and the limited eco-friendly alternatives to deal with the biotic stress it causes. In the present work, water-extractable fractions obtained from vermicompost (WSFv), vermicompost enriched with Trichoderma asperellum (WSFta) and T. virens (WSFtv) were tested as biotechnological tools to reduce the impacts of RKN on gas exchange, water use efficiency (WUE) and nutrient concentration in tomato and bell pepper plants. The plants were infected with 5000 eggs and eventual J2 of RKN and then treated with the water-extractable fractions for seven weeks. It was observed that the addition of WSFta, WSFtv and WSFv increased the CO2 assimilation, stomatal conductance and WUE in the tomato plants. In the bell pepper plants, WSFta, WSFtv, WSFv increased the stomatal conductance, while WUE was higher in the treatment with WSFtv. In fact, the parameters associated with the gas exchange were usually higher in the bell pepper than in the tomato plants. Overall, higher contents of N, Mg, B and Mn were detected when the extracts were applied in both bell pepper and tomato plants. The application of the water-extractable fractions, inoculated or not with Trichoderma, attenuates the RKN damage on the gas exchange parameters and successfully enhanced the nutrient concentration in the infected tomato and bell pepper plants, showing that it could be an important and promising tool for reducing the damage caused by this pathogen. We suggest that both the tomato and pepper plants can cope with the dilemma between growth and stress response via stomata regulation that are modulated by the WSF and Trichoderma.

RNAi suppression of xylan synthase genes in wheat starchy endosperm

The xylan backbone of arabinoxylan (AX), the major cell wall polysaccharide in the wheat starchy endosperm, is synthesised by xylan synthase which is a complex of three subunits encoded by the GT43_1, GT43_2 and GT47_2 genes. RNAi knock-down of either GT43_1 or all three genes (triple lines) resulted in decreased AX measured by digestion with endoxylanase (to 33 and 34.9% of the controls) and by monosaccharide analysis (to 45.9% and 47.4% of the controls) with greater effects on the amount of water-extractable AX (to 20.6 and 19.9% of the controls). Both sets of RNAi lines also had greater decreases in the amounts of substituted oligosaccharides released by digestion of AX with endoxylanase than in fragments derived only from the xylan backbone. Although the GT43_1 and triple lines had similar effects on AX they did differ in their contents of soluble sugars (increased in triple only) and on grain size (decreased in triple only). Both sets of transgenic lines had decreased grain hardness, indicating effects on cell wall mechanics. These results, and previously published studies of RNAi suppression of GT43_2 and GT47_2 and of a triple mutant of GT43_2, are consistent with the model of xylan synthase comprising three subunits one of which (GT47_2) is responsible for catalysis with the other two subunits being required for correct functioning but indicate that separate xylan synthase complexes may be responsible for the synthesis of populations of AX which differ in their structure and solubility.

Depletion of Soil Water-Extractable Organic Matter With Long-Term Coverage by Impervious Surfaces

Water-extractable organic matter (WEOM) in soil is the critical substrate that fuels microbial-driven biogeochemical cycles. However, questions remain regarding whether and how expanding impervious surface area under global urbanization may alter soil WEOM cycling. Based on absorbance and fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we compared the content and chemical signatures of soil WEOM under impervious surfaces with those in adjoining open areas and evaluated the impacts of types (complete sealing by concrete and partial sealing by house structures) and durations (1.5, 27, and 114 years) of impervious surface coverage. The content of soil WEOM and its chromophoric and fluorescent fractions were not significantly changed (less than 20%) after 1.5 years of coverage by concrete and house structures. However, these parameters decreased by more than 30% with 27 and 114 years of coverage by the residential home structures. The microbial-humic-like and protein-like fluorescent WEOM persisted preferentially over the terrestrial-humic-like and nonfluorescent WEOM. FT-ICR MS results suggest various degrees of depletion of biochemical groups in WEOM. While the water-extractable lipid-like compounds increased with 1.5 years of coverage, all studied biochemical groups were depleted with long-term coverage, which might reduce the microbial processing of suberin-derived compounds. This study highlights the remarkable impacts of soil sealing on reducing substrate availability for microbial carbon processing in urban environments.