Evaluating drivers of environmental change in a lake sediment core: Insights from spectroscopic metrics of water-extractable organic matter and stable carbon isotopes

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.

Water extractable organic matter (WEOM) derived from fresh- or early-stage decomposing soil amendment materials may play an important role in the process of organic matter accumulation. In this study, eight WEOM samples extracted with a 40:1 (v/w) water to sample ratio from alfalfa (Medicago sativa L.), corn (Zea mays L.), crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa L.), lupin (Lupinus albus L.), soybean (Glycine max L. Merr.), wheat (Triticum aestivum L.), and dairy manure were investigated using ultraviolet (UV)–visible, Fourier transform infrared (FT-IR), solution 31P nuclear magnetic resonance (NMR), and solid state 13C NMR spectroscopies. UV–visible and FT-IR spectra of the plant-derived WEOM samples were typical for natural organic matter, but possessed less humic-like characteristics than dairy manure-derived WEOM. Solution 31P NMR spectra indicated that WEOM samples extracted from alfalfa, corn, and soybean shoots contained both orthophosphate and monoester P. Of the monoester P in WEOM from soybean shoot, 70% was phytate P. WEOM from crimson clover, hairy vetch, lupin, and wheat shoots contained orthophosphate only. The solid-state 13C NMR spectra of the seven plant-derived WEOM samples indicated that they all were primarily composed of sugars, amino acids or peptides, and low molecular mass carboxylic acids. Carbohydrates were dominant components with very few aromatics present in these samples. In addition, WEOM from crimson clover and lupin, but not other three leguminous plant WEOM samples, contained significant asparagine. On the other hand, WEOM from corn and wheat contained less amino acids or peptides. The spectra of WEOM of dairy manure revealed the presence of significant amounts of nonprotonated carbons and lignin residues, suggesting humification of the manure-derived WEOM. Significant carbohydrates as well as aromatics were present in this WEOM. The P and C bonding information for these WEOM samples may be useful for understanding the effects of WEOM on soil nutrient availability to plants.

Dam decommissioning (DD) is used to solve economic and social problems posed by old dams. However, we ignore the effect of DD on the content and reactivity of large stocks of organic matter (OM) buried in reservoir sediments. We explored temporal changes in the content and reactivity of sediment OM during the first 580 days after the drawdown phase of DD of a large reservoir in the N Iberian Peninsula. We determined the content of sediment OM as organic carbon (OC) in bulk sediment OM and water-extractable OM (WEOM). We estimated the reactivity of bulk sediment OM as its respiration rate and carbon-to-nitrogen ratio, and that of sediment WEOM as its respiration rate, percent biodegradable dissolved OC (%BDOC), and SUVA254. The content of bulk sediment OM was 84 ± 5.1 (mean ± SE) mg OC/ g dry sediment, comparable to the values in the literature on sediment OM in dry sediments from lentic, but higher than in lotic ecosystems. The content of sediment WEOM was 0.81 ± 0.05 mg DOC/g dry sediment, higher than the values in the literature on sediment WEOM from lakes, soils, and rivers. On average, 41 % of WEOM was consumed by microorganisms in two days of incubation, showing the great reactivity of this OM fraction. The content of bulk sediment OM and the respiration rate of WEOM showed a nonlinear temporal trend, while %BDOC increased linearly with sediment exposure time. The labile OM produced by the vegetation that rapidly recolonized the reservoir and photoreactions may explain the temporal increase in %BDOC. Our results suggest that exposed sediments can be a source of labile OM and high C emissions in the river reach downstream of the reservoir after DD.

The subalpine ecosystems of the Bieszczady Mountains are characterized by a mosaic of blueberry shrubs (Vaccinium myrtillus) and tall-grass vegetation, with significant implications for soil organic matter (SOM) dynamics. This study explores how vegetation type influences the content and spectroscopic properties of water-extractable organic matter (WEOM) in the topsoil horizons (O and A) in this region. WEOM is a crucial, bioavailable component of SOM that plays a significant role in nutrient cycling and carbon sequestration, particularly in sensitive mountain ecosystems.Samples of topsoil horizons (O and A) were collected from 20 sites dominated by blueberry shrubs or tall-grass vegetation. Water extracts were analyzed to determine WEOC and WETN concentrations using TOC analyzers. The chemical properties of WEOM were characterized via FTIR-ATR spectroscopy and UV-Vis spectrophotometry. Specific ultraviolet absorbance (SUVA254) and absorbance ratios (E2/E4, E2/E6, and E4/E6) were calculated to assess the aromaticity and molecular composition of WEOM.The O horizons of soils under blueberry shrubs exhibited significantly higher WEOC concentrations compared to those under tall-grass vegetation. However, WETN concentrations were not significantly different between vegetation types. The WEOC/WETN ratio was higher in soils under blueberry shrubs, indicating more carbon-rich WEOM in these areas.Spectroscopic analyses revealed notable differences in WEOM composition. FTIR spectra showed more pronounced bands associated with aliphatic compounds and carboxylic groups in WEOM from shrub-dominated soils, suggesting a higher proportion of less decomposed organic matter. UV-Vis spectroscopy indicated higher SUVA254 values for WEOM in grass-dominated soils, reflecting greater aromaticity and advanced decomposition. In the A horizons, differences in WEOC and WETN concentrations and WEOM properties were minimal, likely due to microbial homogenization and reduced vegetation influence.The results highlight how vegetation significantly affects WEOM quantity and quality, especially in the organic-rich O horizons. Soils under blueberry plants exhibit higher WEOC concentrations and carbon-dominated WEOM, which may improve carbon retention and slow decomposition rates. In contrast, tall-grass vegetation helps to produce more aromatic WEOM, indicative of advanced microbial processing. These findings suggest that shrubification, driven by climate change, can influence WEOM composition and stocks, with implications for carbon cycling and nutrient dynamics in subalpine ecosystems.This study emphasizes the importance of vegetation type as a key determinant of WEOM properties, shaping both the storage and bioavailability of nutrients in mountain soils. These insights are essential for effective vegetation management and the preservation of ecological functions in fragile subalpine zones.

The objectives of this study were to evaluate the binding behavior of Cu(II) and Eu(III) with water extractable organic matter (WEOM) in soil, and assess the competitive effect of the cations. Excitation-emission matrix (EEM) fluorescence spectrometry was used in combination with parallel factor analysis (PARAFAC) to obtain four WEOM components: fulvic-like, humic-like, microbial degraded humic-like, and protein-like substances. Fluorescence titration experiments were performed to obtain the binding parameters of PARAFAC-derived components with Cu(II) and Eu(III). The conditional complexation stability constants (logKM) of Cu(II) with the four components ranged from 5.49 to 5.94, and the Eu(III) logKM values were between 5.26 to 5.81. The component-specific binding parameters obtained from competitive binding experiments revealed that Cu(II) and Eu(III) competed for the same binding sites on the WEOM components. These results would help understand the molecular binding mechanisms of Cu(II) and Eu(III) with WEOM in soil environment.

Lake organic matter is one of the important forms of terrestrial carbon, and its sedimentary evolution is affected by many factors such as climate and sources. However, few studies have been conducted on the feedback mechanism of the sedimentary evolution of organic matter to climate change in cold and arid lakes. Historical variations and compositions of sources of the sediment organic matter (SOM) of Hulun Lake, a typical lake in the cold and arid region of China, were studied by multiple methods. The interactions and fee7dback mechanisms between the sedimentary evolution of SOM and climate change, and compositions of SOM source change, were also discussed. Overall, the characteristic indexes of the SOM, including total organic carbon (TOC), carbon stable isotope (δ13C), carbon to nitrogen ratio (C/N), and fluorescence intensity (FI) of the protein-like component in water extractable organic matter (WEOM), showed obvious and uniform characteristics of periodical changes. The indexes were relatively stable before 1920, and fluctuated from 1920 to 1979. Since the 1980s, values of TOC, δ13C, and FI of the protein-like component in WEOM has increased, while C/N decreased. The absolute dominant contribution of terrestrial source to the SOM had changed, and the relative average contribution rate of autochthonous source increased from 17.6% before 1920 to 36.9% after 2000. The increase of temperature, strong evaporation concentration effect, and change of compositions of SOM sources are the important driving factors of the sedimentary evolution of organic matter in Hulun Lake.

Carbon and nitrogen isotopes in sediments collected from the Manwan and Dachaoshan Reservoirs, which were created by cascade dams along the Lancang–Mekong River, were measured to preliminarily analyse their organic matter sources. Sediment samples were collected from the Manwan and Dachaoshan Reservoirs twice during both the rainy and dry seasons by using a gravitational bottom sampler. One set of samples was collected from the river centres of each reservoir, and the other set was collected from the river edges, 5 m from each bank. Sediments were divided by wet sieving into three classes of water-stable aggregates based on sizes, including macroaggregates, microaggregates, and silt and clay. At the same time, terrestrial plants (Phragmites) and surface soil samples (0–5 cm) were collected from the river bank. The total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and stable nitrogen isotope (δ15N) were measured for sediments, soils, and plants to preliminarily analyse the sources of organic matter in the sediments. Because of the presence of dams, a greater amount of the fine fraction was deposited in the reservoirs, which adsorbed and preserved more organic matter. Although terrestrial sources of organic matter dominate in riverine systems, aquatic sources have been shown to act as new organic matter sources in the Manwan Reservoir. Input from C3 plants was the major new organic matter source in the Dachaoshan Reservoir, which also had strong inputs from aquatic sources. In the dry season, C3 plant input was significantly reduced and less fine fraction was found in the sediments. The high nitrogen isotope values of the sediments collected during the dry season indicated that strong organic matter decomposition decreased the storage of organic matter. The cascade dams permanently changed the aquatic environment and modified regional carbon flows resulting in the diversification of organic matter sources. The major source of sedimentary organic matter was terrestrial, including from plants, soils, and particle organic matter, with the new organic matter absorbed by macroaggregates primarily stemming from aquatic organisms and C3 plants. Samples collected during the dry season exhibited strong decomposition, which further weakened the preservation of organic carbon in sediments.

Effect of Temperature, pH and Salt on Fluorescent Quality of Water Extractable Organic Matter in Black Soil

The dynamics of water extractable organic matter (WEOM) in common arable topsoils: I. Quantity, quality and function over a three year period

The dynamics of water extractable organic matter (WEOM) in common arable topsoils: II. Influence of mineral and combined mineral and manure fertilization in a Haplic Chernozem

Dissolved and water-extractable organic matter in soils: a review on the influence of land use and management practices

Dissolved organic matter is the most mobile part of soil organic matter. At the same time, its change and transformation processes occuring during soil erosion have not been sufficiently studied. The goal of the work was to assess the optical properties of water-extractable organic matter (WEOM) in arable soils of different degree of degradation from erosion and sedimentation in a plowed small arable catchment in the Kursk region. We studied WEOM of arable Protocalcic Chernozems (noneroded and moderately eroded) and their analogue with soil matter sedimentation – Novic Protocalcic Chernozems. WEOM was isolated from aggregates 2–1 and 10 mm. Aqueous extracts were characterized by their organic carbon and nitrogen content. Optical properties were assessed based on absorption spectra and three-dimensional fluorescence spectra. It was shown that in terms of the main quantitative indicators of soil organic matter – the content of organic carbon and nitrogen, as well as the pH value – washed away and reclaimed soils were close to each other and differed significantly from Protocalcic Chernozems. At the same time, both the quantitative and qualitative indicators of WEOM showed a different trend: the WEOM of Novic Protocalcic Chernozems differed significantly from noneroded and moderately eroded Protocalcic Chernozems. Besides, some indicators of WEOM (nitrogen content, SUVA254, S350–400 и SR) depended on the size of the aggregates from which WEOM was obtained (2–1 or 10 mm). In addition, the fluorescent properties of WEOM depend on the size of the aggregates. The obtained data allow us to conclude that the properties of WEOM in a small arable catchment in the central forest-steppe zone are largely determined by the processes of destruction of non-water-stable aggregates and the consolidation of their particles, as well as the leaching of water-soluble organic matter. When aggregates are destroyed by water, their particles migrate with flows along the slope, and organic matter undergoes decomposition; in depressions, particles accumulate, consolidate into blocky structural units, while the properties of their WEOM change significantly, both due to the degradation of organic matter and as a result of its leaching.

Changes in water extractable organic matter (WEOM) in a calcareous soil under field conditions with time and soil depth

Water-extractable organic matter (WEOM) plays an important role in many chemical processes and in soil organic matter accumulation. Large amounts of WEOM can be released by green manure when being incorporated into the soil. However, the characteristics of WEOM extracted from different green manures (GMs) are unclear. In this study, WEOM samples were extracted from Chinese milk vetch (Astragalus sinicus L), radish (Raphanus sativus L), ryegrass (Lolium perenne L), hairy vetch (Vicia villosa L), February orchid (Orychophragmus violaceus) and rye (Secale cereale L) and their characteristics were studied by spectroscopic analysis. WEOM generated from legume GMs contained more water-extractable organic carbon (WEOC) than other GMs and Chinese milk vetch was the highest. UV-visible spectroscopy analysis revealed that all the samples were rich in C=C and C=O functional groups. Carboxylic acid, alcohol, phenol, fatty acids, aliphatic aromatic and amide compounds were found by FTIR spectroscopy in these WEOM samples. WEOM derived from Chinese milk vetch contained more carboxylic acid and inorganic sulphates only appeared in WEOM extracted from the radish. The aromaticity index, humification index, and FTIR absorption ratio 1650/2925 and 1650/2850 showed that WEOM extracted from Chinese milk vetch and hairy vetch had a relatively lower aromaticity and humification degree than other GMs. The fluorescence regional integration (FRI) analysis showed that Chinese milk vetch and hairy vetch had a higher portion of protein-like substance fractions than other GMs. We may deduce that WEOM generated from Chinese milk vetch and hairy vetch has higher chemical activity than other GMs.

Effect of ecosystem type and fire on chemistry of WEOM as measured by LDI-TOF-MS and NMR