Composition Of Water-extractable Organic Matter Research Articles

Strong associations between dissolved organic matter and microbial communities in the sediments of Qinghai-Tibetan Plateau lakes depend on salinity

In aquatic ecosystems, dissolved organic matter (DOM) plays a vital role in microbial communities and the biogeochemical cycling of elements. However, little is known about the associations between DOM and microbial communities in lake sediments. This study investigated the composition of water-extractable organic matter and microbial communities in surface sediments of lakes with different salinities on the Qinghai-Tibet Plateau. Ultrahigh-resolution mass spectrometry and high-throughput microbial sequencing techniques were employed to assess the associations between molecular diversity and microbial diversity and the effects of salinity in 19 lakes spanning a salinity range from 0.22 ‰ to 341.87 ‰. Our results show that increasing salinity of lake water led to higher molecular diversity of DOM in surface sediments. High-salinity lakes exhibited distinct DOM characteristics, such as lower aromaticity, smaller molecular weight, and higher oxidation degree, compared to freshwater lakes. The complexity of the microbial network composition of sediments first increased and then decreased with the increase of salinity. Moreover, as salinity increases, the dominant species transitioned from Gammaproteobacteria to Bacteroidia, and this transition was accompanied by a decrease in microbial diversity and an increase in molecular diversity. Microbial factors accounted for 34.68 % of the variation in the molecular composition of DOM. Overall, this study emphasizes the significant effects of salinity on both molecular and microbial diversity in lake sediments. Furthermore, our findings underscore the importance of microbes in controlling the range of organic compounds present in lakes and deepen our knowledge of the biogeochemical cycling of DOM.

Heat-induced changes in soil water-extractable organic matter characterized using fluorescence and FTIR spectroscopies coupled with dimensionality reduction methods

Water extractable organic matter (WEOM) is a very mobile and reactive soil OM fraction, critical in the translocation of carbon (C) from soils to other environmental compartments. Transformations of WEOM due to soil heating can have implications not only at a local scale, but in places far away from the location of the event. However, their accurate characterization is costly when analyzing a large number of samples. The objectives of this work were to identify common patterns for the changes in WEOM caused by the heating of various soil types using a combination of spectroscopic and dimensionality reduction techniques. Six soils from Spain, Kenya and Israel were collected at depths 0–10 cm and analysed before and after heating in air to temperatures of 300 and 600 °C. Fluorescence EEMs were measured in soil–water extracts containing WEOM, and decomposed using parallel factor (PARAFAC) analysis. The FTIR spectra were measured in freeze-dried extracts and further analysed using non-negative matrix factorization (NMF). Total organic C and SUVA254 values of the extracts experienced changes with the heating treatments that were soil dependant. Four PARAFAC and three NMF components were sufficient to characterize WEOM changes in all soils, which showed common thermal transformation patterns irrespective of their origin and properties. Thermal transformation of fluorescent WEOM led to the increase in the proportion of a component with an emission maximum at Ex 300/Em 392 nm, and to a lower extent one with the emission maximum at Ex 300/Em 426 nm. Concomitantly, the proportion of components with emission maxima at longer excitation wavelengths was reduced. These changes occurred at the lowest heating temperature and were maintained at 600 °C, and they seem to indicate a depletion of fluorescent components more conjugated, bigger in size, and an enrichment in smaller ones. The NMF components obtained from FTIR spectra showed an increase of the proportion of compounds with CO bonds, more oxidized. No correlations were found between the components obtained with each method, thus indicating that the information obtained from the fluorescence EEMs-PARAFAC analysis and the NMF decomposition of FTIR spectra is complementary. It can be concluded that there is a common pattern of WEOM changes induced by thermal soil transformations irrespective of the origin and properties of the soils studied, and that the combination of different spectroscopic techniques coupled with dimensionality reduction methods can be used as a simple and low-cost method to fingerprint changes in WEOM composition, in general, and those caused by soil heating, in particular.

Organic matter in aqueous soil extracts: Prediction of compositional attributes from bulk soil mid-IR spectra using partial least square regressions

Water-extractable organic matter (WEOM) is labile and a key component of soil organic matter. Thus, the prediction of soil WEOM concentration and composition-related characteristics is of great interest. The main objective of this study was to model and predict dissolved organic C (DOC) concentration and WEOM composition-related attributes of aqueous soil extracts using mid-infrared (IR) spectra of bulk soils coupled with partial least square (PLS) regression. Absorbance of UV light at 254 nm (Abs254), considered proportional to the concentration of aromatic substances in soil extracts, and light emission intensities proportional to concentrations of some components controlling WEOM fluorescence, were used as composition-related attributes of the soil extracts. The DOC-normalized Abs254 and emission intensities were used as composition-related attributes of WEOM. Application of PLS regressions for predicting spectroscopy-based composition-related attributes of aqueous soil extracts, using bulk soil IR spectra, is novel. Mid-IR spectra were determined for 216 soil samples collected from different (i) Israeli climate regions (Mediterranean and Semi-arid), (ii) types of land use (field crops, orchard and non-cultivated land), (iii) two depths (0–10 and 30–60 cm), (iv) sampling seasons (Fall and Spring). Prediction of DOC concentrations in the soil extracts was of limited and variable success evaluated by the coefficient of determination and slope of the linear regression of predicted vs measured values, with some soil subsets yielding no satisfactory predictions. However, Abs254 and emission intensities of fluorescent humic-like components were predicted more successfully than DOC concentrations, suggesting that WEOM aromatic and fluorescent components are better presented in soil IR spectra compared to WEOM aliphatic substances. Yet, prediction of the specific UV absorbance (SUVA), using bulk soil IR spectra, was less successful than prediction of Abs254. Prediction of DOC-normalized emission intensities of fluorescent components (analogous to SUVA in fluorescence spectroscopy) was not successful. The differences between the success of predicting extract properties, i.e., Abs254 and fluorescence emission intensities, and the prediction of DOC-normalized derivatives, suggest that concentrations of aromatic (fluorescent) components in soil extracts are better predicted using PLS regression analysis of bulk soil mid-IR spectra than the WEOM composition.

Bulk and water-extractable organic matter from compost: evaluation of the selective dissolution in water using infra-red absorbance ratios.

Abstract Land application of composts affects concentration and composition of dissolved organic matter (OM) which plays important roles in soil functioning and may have effects on spreading of environmental pollution. Linking between the compositions of bulk compost OM and its water-soluble fraction may, therefore, allow better understanding and prediction of environmental impact of compost land use. The objectives of this study were to (i) examine compositional links between bulk compost OM and its water-extractable OM (WEOM), and (ii) evaluate and quantify selectivity of bulk compost OM dissolution, based on infra-red (IR) absorbing functional groups. For that, 8 different composts and their freeze-dried WEOMs were characterized by mid-IR transmission spectroscopy. Compositions of compost OM and WEOM were characterized in terms of ratios (R) defined on the basis of both areas and heights of specific IR bands in relation to absorbance by aliphatic CH groups. A simple novel approach is suggested, whereby selective dissolution of compost OM components is quantified by relating the R values determined for WEOM to those associated with compost OM. Significant similarities of IR spectra found in a series of WEOMs (and, to a lesser extent, in a series of compost OMs) suggest significant contributions of OM carboxylic groups to various bands. WEOM composition (characterized by the R values) is associated, to a certain extent, with compost OM composition expressed by similar type indices. The WEOM aromaticity estimated by specific UV absorbance correlated strongly with some R values determined for bulk OM.

Assessing the stability in composting of penicillin mycelial dreg via parallel factor (PARAFAC) analysis of fluorescence excitation–emission matrix (EEM)

Four composting treatments with different ratios of penicillin mycelial dreg (PMD) to sewage sludge (T-1, 1:0.4; T-2, 1:0.8; T-3, 1:1.2; T-4, 0:1) were investigated to characterize the property and composition of water-extractable organic matter (WEOM) and assess their stabilization via parallel factor analysis (PARAFAC) of fluorescence excitation–emission matrix (EEM). The results show that DOC, DON, DOC/DON ratio and respirometric rate decrease; SUVA254 increase in four treatments. Penicillin degradation occurs mainly in the first 12days. A five-component model was obtained, in which three protein-like and two humic-like components were identified. The maximum fluorescence intensity (Fmax) of components is affected by the initial composition of treatments. Furthermore, the relationships between the WEOM parameters and the Fmax of components in T-2 were established using Pearson correlation analysis, indicating that component 3, component 4 and component 5 are the effective parameters in evaluating the stability for composting PMD. To sum up, EEMs-PARAFAC can be proposed to evaluate stability during composting of PMD.

Shifts in microbial community and water-extractable organic matter composition with biochar amendment in a temperate forest soil

Biochar amendment in soil has been proposed as a carbon sequestration strategy which may also enhance soil physical and chemical properties such as nutrient and water holding capacity as well as soil fertility and plant productivity. However, biochar may also stimulate microbial activity which may lead to increased soil CO2 respiration and accelerated soil organic matter (OM) degradation which could partially negate these intended benefits. To investigate short-term soil microbial responses to biochar addition, we conducted a 24 week laboratory incubation study. Biochar produced from the pyrolysis of sugar maple wood at 500 °C was amended at concentrations of 5, 10 and 20 t/ha in a phosphorus-limited forest soil which is under investigation as a site for biochar amendment. The cumulative soil CO2 respired was higher for biochar-amended samples relative to controls. At 10 and 20 t/ha biochar application rates, the concentration of phospholipid fatty acids (PLFAs) specific to Gram-positive and Gram-negative bacteria as well as actinomycetes were lower than controls for the first 16 weeks, then increased between weeks 16–24, suggesting a gradual microbial adaptation to altered soil conditions. Increases in the ratio of bacteria/fungi and lower ratios of Gram-negative/Gram-positive bacteria suggest a microbial community shift in favour of Gram-positive bacteria. In addition, decreasing ratios of cy17:0/16:1ω7 PLFAs, a proxy used to examine bacterial substrate limitation, suggest that bacteria adapted to the new conditions in biochar-amended soil over time. Concentrations of water-extractable organic matter (WEOM) increased in all samples after 24 weeks and were higher than controls for two of the biochar application rates. Solution-state 1H NMR analysis of WEOM revealed an increase in microbial-derived short-chain carboxylic acids, lower concentrations of labile carbohydrate and peptide components of soil OM and potential accumulation of more recalcitrant polymethylene carbon during the incubation. Our results collectively suggest that biochar amendment increases the activity of specific microorganisms in soil, leading to increased CO2 fluxes and degradation of labile soil OM constituents.

Soil Aggregate Destruction by Ultrasonication Increases Soil Organic Matter Mineralization and Mobility

Ultrasonication is widely used in soil organic matter (SOM) fractionation studies to break up soil aggregates to disperse the soil into its primary particles. An increasing number of studies also aim to incubate SOM fractions obtained by physical soil fractionation to study the bioavailability of different soil organic C (SOC) pools. To evaluate possible influences of ultrasonic soil disruption on short-term SOM bioavailability as well as the content and composition of water-extractable organic matter (WEOM) and salt-extractable (10 mM K2SO4) organic matter (SEOM), we conducted a laboratory incubation experiment with aggregated soil material from an Ap horizon of a cropland soil. Bulk soil and subsamples in which aggregates had been disrupted by ultrasonication were incubated in triplicate for 18 d at 20°C. During the incubation, CO2 production was measured continuously. At the beginning and at the end of the experiment, the chemical composition of SEOM was analyzed for the different experimental variants by ultraviolet (UV)-absorbance and solid state 13C cross polarization magic angle spinning nuclear magnetic resonance (13C-CPMAS NMR) spectroscopy. Additionally, we studied the effects of WEOM removal on heterotrophic soil respiration to assess artifacts by unavoidable WEOM removal during soil wet sieving, which is an important step in soil fractionation. After repeated leaching of unsonicated and sonicated soil samples, 0.5 to 1.0% of the total organic C (OC) pool was associated with the WEOM fraction. Ultrasonic disruption of soil aggregates resulted in increased amounts and changed composition of SEOM as well as in a 27% increase in CO2 production compared with intact soils. Incubation of ultrasonicated samples resulted in decreased O/N alkyl C and increased aromatic C contents in SEOM, indicating mineralization of potentially mobile organic matter (OM). Ultrasonic treatment during soil fractionation leads to enhanced dissolved OM (DOM) leaching and increased SOM bioavailability, which may bias results from incubation experiments on separated SOM fractions.

Changes in the chemical composition of water-extractable organic matter during composting: Distribution between stable and labile organic matter pools

Aerobic decomposition and stabilization of organic matter during the composting of waste materials is primarily due to the biochemical transformation of water-soluble compounds in the liquid phase by the microbial biomass. For this reason water-soluble organic matter represents the most active fraction of compost, both biologically and chemically, and thus should directly reflect the biochemical alteration of organic matter. This work aims to elucidate the microbial-mediated processes responsible for the distribution of soluble organic matter between stable and labile pools with composting time. Accordingly, chemical analysis as well as UV absorption, and 1H and 13C-NMR spectroscopy of samples collected during the industrial composting of urban waste revealed microbial induced transformation of water-extractable organic matter over time. The chemical composition changed from labile, hydrophilic, plant-derived organic compounds in the beginning to predominately stable, hydrophobic moieties comprising lignin-derived phenols and microbially-derived carbohydrates at later stages of composting.