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.

Characterization and function of particulate organic matter: Evidence from lakes undergoing ecological restoration

Particulate organic matter (POM) plays a crucial role in the organic composition of lakes; however, its characteristics remain poorly understood. This study aimed to characterize the structure and composition of POM in Lake Baiyangdian using many kinds of techniques and investigate the effects of different extracted forms of POM on water quality. The suspended particulate matter in the lake had complex compositions, with its components primarily derived from aquatic plants and their detritus. The organic matter content of the suspended particulate matter was relatively high (organic carbon content 27.29–145.94 g/kg) for the sum of three extractable states (water-extracted organic matter [WEOM], humic acid, and fulvic acid) and one stable bound state (humin). Spatial distribution analysis revealed that the POM content in the water increased from west to east, which was consistent with the water flow pattern influenced by the Baiyangdian water diversion project. Fluorescence spectroscopy analysis of the WEOM showed three prominent peaks with excitation/emission wavelengths similar to those of dissolved organic matter peaks. These peaks were potentially initial products of POM conversion into dissolved organic matter. Furthermore, the intensity of the WEOM fluorescence peak (total fluorescence peak intensity) was negatively correlated with the inorganic nitrogen concentration in water (p < 0.01), while the intensity of the HA fluorescence peak showed a positive correlation with the inorganic nitrogen concentration (p < 0.01). This suggested that exogenous organic matter inputs led to the diffusion of alkaline dissolved nitrogen from sediment into water, while degradation processes of aquatic plant debris contributed to the decrease in inorganic nitrogen concentrations in the water column. These findings enhance our understanding of POM characteristics in shallow lakes and the role of POM in shallow lake ecosystems.

The specific molecular signature of dissolved organic matter extracted from different arctic plant species persists after biodegradation

Dissolved organic matter (DOM) is a small but very reactive pool of organic matter (OM) in the environment. Its role is related to its composition, which depends on its source. In soils, vegetation is the main source of DOM, and biodegradation is the main regulating mechanism. This study aims to characterise DOM produced by contrasted arctic vegetation species and their biodegradation products.The water-extractable organic matter (WEOM) was produced from C. stellaris (lichen), E. vaginatum (sedge), A. polifolia (dwarf evergreen shrub) and B. nana (deciduous dwarf shrub). The WEOM were inoculated with a common aerobic heterotrophic soil bacteria (P. aureofaciens) and incubated for 7 days. During the experiment, WEOM was characterised through a wide range of analytical methods (TOC, UV–Vis absorbance, high-performance ion chromatography and HRMS Orbitrap).The results showed bacteria consumed a significantly greater proportion of WEOM produced by C. stellaris than by A. polifolia and B. nana at the end of the experiment (p < 0.05). Furthermore, the number of features in WEOM decreased for C. stellaris and E. vaginatum, whereas it increased for B. nana. These findings shed light on the species-specific biodegradation processes that rely on the initial composition of DOM, specifically influenced by the vegetation's capacity to produce recalcitrant compounds. Furthermore, our results emphasised that even though bacterial activity greatly impacted molecular characteristics, the WEOM produced by different vegetation species maintained their distinct molecular signatures. As a result, it can be inferred that the DOM found in natural environments directly reflects the relevant vegetation cover despite the strong influence of biogeochemical processes on DOM molecular composition. This should be considered when developing models to assess the influence of climate change on vegetation cover composition and its subsequent effects on DOM dynamics in soil and surface waters.

Effects of long-term application of organic materials on soil water extractable organic matter, fulvic acid, humic acid structure and microbial driving mechanisms

Effects of long-term application of organic materials on soil water extractable organic matter, fulvic acid, humic acid structure and microbial driving mechanisms

Spatial variation of soil organic matter and metal mobility in wetland soils: Implications for biogeochemical processes in lateritic landscape

The evolution of the lateritic landscape over time changes the path of pedogenesis and soil hydrology. The occurrence of upland wetlands in the savannah flat plateau transforms well-drained lateritic soils into hydric soils enriched in organic matter. We investigated the composition of soil organic matter (SOM) produced in a wetland and their link with the mineral constituents of soil and their contribution to the mobility of Si, Fe, and Al in the soil. Six tranches were opened in a soil catena located from the plateau toward the center of the wetland, representing different hydromorphic features. Soil morphology was described, and chemical, physical and mineralogical analyses were performed to evaluate soil properties. Chemical fractionation of Soil Organic Matter (SOM) was performed and total organic carbon (TOC), dissolved organic carbon (DOC), particulate organic carbon (POC), and water-extractable organic matter (WEOM) were determined in the soil horizons. Metal complexing properties of DOM were determined using an excitation-emission matrix of fluorescence. The occurrence of Fe eluviation in the superficial horizons, Fe2O3 enrichment in the mottled horizons, and ferricrete formation in the center of the wetland were observed. Geochemical transfer assessment and the variation of oxides within the horizons showed that Fe2O3 and SiO2 were more mobile than Al2O3, whose variability in the gley horizon reflected the mobility of Fe2O3 and SiO2. The stability constants of Al and Cu showed an increase with the depth for WEOM and SOM. The pattern of the organic carbon fraction varied according to the level of hydromorphy and soil depth. It was also observed an increase in the C/N ratio with depth, possibly related to the reduced bioavailability of DOC due to its adsorption with soil solids.

Chemical Composition of the Aboveground Tissues of Miscanthus × giganteus and Relationships to Soil Characteristics

AbstractTo reduce the C footprint of human activities, there is a growing need for alternative energy sources including the production of bioenergy feedstocks. Miscanthus × giganteus is a high yielding grass with low environmental impact and high potential for feedstock use. Studying the composition of the aboveground tissues of Miscanthus is important for understanding feedstock quality for biofuel conversion and how crop residue quality may affect soil input management. Data on Miscanthus leaf and stem chemistry including carbon (C), nitrogen (N), macronutrient concentrations, and the optical characteristics of the water extractable organic matter (WEOM) was analyzed to identify differences in composition between aboveground tissues and modeled to identify soil variables that may be correlated with tissue chemistry. Leaves and stems were dominated by N, potassium (K), calcium (Ca), phosphorus (P), and magnesium (Mg), but overall, the leaves contained higher nutrient concentrations compared to the stems. The leaves displayed elevated Si:K (0.0935) and Ca:K (0.445) ratios and lower C:N (36) and C:P (323) ratios compared to the stems (0.0560, 0.145, 150, and 645, respectively). Leaf WEOM contained large, aromatic, and complex structures, while the stem WEOM was dominated by small, recently produced structures. Varying relationships were found between tissue C and the mobile C pool in surface (0–15 cm) and deep (45–60 cm) soils. Overall, Miscanthus leaves had a chemical composition indicative of reduced biofuel quality compared to the stems. The relationships with soil mobile C suggest a dynamic linkage between Miscanthus physiology and this active soil C pool. These results have implications for crop nutrient allocation and nutrient management practices.

Bentonite addition enhances the biodegradation of petroleum pollutants and bacterial community succession during the aerobic co-composting of waste heavy oil with agricultural wastes

Soil contamination with petroleum significantly threatens the ecological equilibrium and human health. In this context, aerobic co-composting of waste heavy oil with agricultural wastes was performed in the present study to remediate petroleum pollutants through four treatments: CK (control), T1 (petroleum pollutant), T2 (petroleum pollutant with bentonite), and T3 (petroleum pollutant with humic acid-modified bentonite). Comprehensive analyses were conducted to determine the physicochemical parameters, enzymatic activities, removal of petroleum pollutants, microbial community structure, and water-extractable organic matter in different composting systems. Structural equation modeling was employed to identify the key factors influencing the removal of petroleum pollutants. According to the results, petroleum pollutant removal percentages of 44.94%, 79.09%, and 79.67% could be achieved with T1, T2, and T3, respectively. In addition, the activities of polyphenol oxidase (51.21 U/g) and catalase (367.91 U/g), which are the enzymes related to petroleum hydrocarbon degradation, were the highest in T3. Moreover, bentonite addition to the treatment increased the nitrate nitrogen storage in the compost from 10.95 mg/kg in T1 to 18.63 and 17.41 mg/kg in T2 and T3, respectively. Humic acid-modified bentonite could enhance the degree of compost humification, thereby leading to a higher-quality compost product. Collectively, these findings established bentonite addition as an efficient approach to enhance the compost remediation of petroleum pollutants.

Arsenic-dissolved organic matter complexation in water soluble extracts from lignite

Arsenic in groundwater is a global threat to public health. Recently, As mobility has been tied to the concentration and chemical characteristics of dissolved organic matter (DOM) through formation of As-DOM complexes. To date, there has been a wide range of DOM types studied to understand As-DOM interactions, but most of these have focused on surface water derived materials and not groundwater DOM. We address this gap in knowledge by simulating groundwater DOM using water extractable organic matter (WEOM) from two lignite deposits and treating the extracts with increasing concentrations of As. As-DOM complexes were measured using size-exclusion chromatography coupled to multiple detectors including an inductively coupled plasma mass spectrometer (ICPMS) for As detection as well as fluorescence and variable wave detectors for organic matter detection. First, we found two different size fractions of As-DOM, one of ∼1 kDa and another of ∼15 kDa, depending on the DOM types. The smaller As-DOM complex (∼1 kDa) was approximately 10 times more abundant than the larger complex (∼15 kDa). Second, we found that the lignite derived DOMs showed higher conditional distribution coefficients than did the surface water reference material (Suwanee River Natural Organic Matter, SRNOM). Finally, the data showed good fit (R2 > 0.92) to one-site ligand binding models, and the lignite derived DOMs showed higher maximum sorbate concentrations (Bmax) compared to SRNOM. Together, this study shows that As-DOM complexation is an important control on As speciation, even in groundwater systems.

Effect of soil microbial community structure on the chemical compositions of different soil organic matter fractions in land uses of the Pearl River Estuary

Understanding the interactions between the dynamics of soil organic matter (SOM) and soil microbial communities is important for predicting the stability and fate of estuarine SOM following anthropogenic disturbance. However, current knowledge on the influence of microbial communities on the chemical compositions of different SOM fractions in estuarine land uses still remains incomplete. In this study, we used high-throughput sequencing, Fourier transform infrared spectroscopy, UV–Vis absorption, and fluorescence spectroscopy to investigate soil microbial communities and characterize the chemical compositions of bulk SOM and water-extractable organic matter (WEOM) across different land uses (artificial forests (AF), farmland (FL), natural wetland (NW), construction land (CL), and abandoned land (AL)) in the Pearl River Estuary. The results showed that the bulk SOM in FL and AL exhibited comparable or higher levels of aromatic functional groups (aromatic CH and aromatic CC) compared to other land uses. The HIX and SUV254 indexes further confirmed that AF and FL had more aromatic and hydrophobic WEOM. Apart from FL, where anthropogenic humus constituted the largest proportion, the WEOM fraction predominantly consist of three components in the following order across all land uses, ordered as anthropogenic humus < microbial humus < protein humus. The chemical compositions of the different SOM fractions showed a stronger correlation with the dominant bacterial phylum composition, especially for the WEOM fraction, compared to the fungal composition. Meanwhile, our findings demonstrate that bacterial diversity showed a stronger relationship with the chemical composition of WEOM compared to abundance. Overall, this study sheds light on the impact of land use on the quantity and quality of SOM fractions, underscoring the varying influence of microorganisms on the chemical compositions of different SOM fractions within estuarine environments.

A comprehensive analysis of soil health indicators in a long‐term conservation tillage experiment

AbstractConservation tillage (CT) is a ploughless tillage with a reduced number of operations, and its positive effect on soil functions and health is widely known. Multivariate analyses are required to choose indicators that adequately characterize the changes in soil health. However, there is little research on the comprehensive analysis of the full spectrum of soil physical, chemical and biological properties. Therefore, we examined 21 soil parameters in a long‐term CT experiment conducted in Hungary. Four pairs of similarly sized CT and conventional ploughing tillage (PT) plots were set up in 2003 on Luvisols. The soil samples were collected after 17 years. The total organic carbon (TOC) increased significantly in the 0–15 cm layer at CT sites compared to those in PT, indicating a total increase of 5.22 t ha−1 TOC stock. In addition, the increasing biological activity and improved soil structure were the most important processes at the CT sites. Furthermore, more complex humic substances with higher molecular weights are characteristic of water‐extractable organic matter (WEOM) as a result of CT. The potentially available nitrogen, phosphorus and potassium were also measured with a relatively high response ratio. Slowly changing parameters, such as cation exchange capacity and base saturation, are important soil physical and chemical parameters, but are not good indicators of the impact of tillage practices. Based on principal component analysis, we suggest the use of water‐extractable organic C, amino‐nitrogen, water‐stable aggregates, available P and K and photometric analysis of WEOM to identify the soil improving processes.

Stable C and N isotope abundances in water-extractable organic matter from air-dried soils as potential indices of microbially utilized organic matter

Stable carbon (C) and nitrogen (N) isotopes (13C and 15N) in water-extractable organic matter (WEOM) derived from air-dried soils may be applicable to elucidate the microbial decomposition of soil organic matter (SOM), which is crucial in terrestrial C cycles. A total of 40 soil samples were collected from a depth of 0–6 cm from a temperate broadleaved forest in Japan with vegetation succession from grassland approximately 150 years ago. Those soil samples were air-dried before the water extraction process and organic matter analysis. The C and N concentrations of WEOM were &amp;lt;3.6% of those of the bulk soil and were positively correlated with those of the bulk soil at a p-value of &amp;lt; 0.01. A positive correlation between the two fractions (i.e., WEOM and bulk soils) was also found for natural 13C and 15N abundances (δ13C and δ15N; p &amp;lt; 0.01). However, the C/N ratio of WEOM was slightly correlated with that of bulk soils, exhibiting a narrow range of values of ~10. Thus, those features of the WEOM were similar to the well-known features of microbial biomass. The δ13C and δ15N enrichments in WEOM relative to bulk soil, the difference in stable isotope abundances between bulk SOM and WEOM were negatively and positively correlated, respectively, with the concentrations of organo-mineral complexes and short-range order minerals (non-crystalline oxyhydroxides of aluminum and iron, allophane, imogolite, and allophane-like constituents), which play significant roles in SOM stabilization in soils. These relationships suggest that the stable isotopic enrichments in WEOM can be a good indicator of the microbial utilization of soil C and N under different substrate availabilities, which are crucial to SOM decomposition and decomposability substantially varying from local to global scales.

Exposed sediments in a temperate-climate reservoir under dam decommissioning contain large stocks of highly bioreactive organic matter

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.

Insights into the formation and stability of soil aggregates in relation to the structural properties of dissolved organic matter from various organic amendments

Organic matter is an important determinant of soil aggregate formation and stability. However, the role of its structure, especially that of dissolved organic matter (DOM) from organic amendments (OAs), in the aggregation process remains imprecise. The purpose of this study was to understand the relationship between the chemical structure of water-extractable organic matter (WEOM), potential source of DOM from OAs, and their aggregate formation/stability functionality through model experiment. The WEOM samples were prepared from bark compost (BC), coffee residue compost (CRC), cattle manure compost (CMC), sewage sludge compost (SSC), fish cake (FC), and rapeseed oil cake (ROC) and characterized using high-performance size exclusion chromatography and 13C nuclear magnetic resonance and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies. An upland field soil was packed in glass columns and each WEOM solution or ultrapure water (Control) was applied repeatedly during a 30-day period. Aggregate size distribution (>2000, 500–2000, 250–500, 53–250, and <53 µm) was then measured and aggregate stability indices, mean weight diameter (MWD), and geometric mean diameter (GMD) were calculated. The addition of WEOM increased the distributions of soil mass and soil organic carbon (C) into the > 2000 µm aggregate fraction compared to Control treatment, and the ROC-, CRC-, and SSC-WEOM treatments resulted in greater MWD and GMD than Control and/or the other WEOM treatments. The % O-alkyl C (29–59% of total C) and the relative abundance of high-molecular-weight (HMW) fraction (>10 kDa; 2.3–7.1% of total) in WEOM correlated positively with the proportion of > 2000 µm aggregate fraction. The relative abundance of HMW fraction also correlated positively with MWD. These findings suggest that the high amounts of polysaccharides, suggested by % O-alkyl C and supported by DRIFT spectra, and/or other HMW components are desirable structural characteristics of WEOM for promoting macroaggregate formation and aggregate stability.

Electrokinetic remediation leads to translocation of dissolved organic matter/nutrients and oxidation of aromatics and polysaccharides

Dissolved organic matter (DOM) in the sediment matrix affects contaminant remediation through consumption of oxidants and binding with contaminants. Yet the change in DOM during remediation processes, particularly during electrokinetic remediation (EKR), remains under-investigated. In this work, we elucidated the fate of sediment DOM in EKR using multiple spectroscopic tools under abiotic and biotic conditions. We found that EKR led to significant electromigration of the alkaline-extractable DOM (AEOM) toward the anode, followed by transformation of the aromatics and mineralization of the polysaccharides. The AEOM remaining in the cathode (largely polysaccharides) was resistant to reductive transformation. Limited difference was noted between abiotic and biotic conditions, indicating the dominance of electrochemical processes when relatively high voltages were applied (1–2 V/cm). The water-extractable organic matter (WEOM), in contrast, showed an increase at both electrodes, which was likely attributable to pH-driven dissociations of humic substances and amino acid-type constituents at the cathode and the anode, respectively. Nitrogen migrated with the AEOM toward the anode, but phosphorus remained immobilized. Understanding the redistribution and transformation of DOM could inform studies on contaminant degradation, carbon and nutrient availability, and sediment structural changes in EKR.

Exploring impacts of water-extractable organic matter on pre-ozonation followed by nanofiltration process: Insights from pH variations on DBPs formation

This study investigated the influence of pH (4–10) on the treatment of water-extractable organic matter (WEOM), and the associated disinfection by-products (DBPs) formation potential (FP), during the pre-ozonation/nanofiltration treatment process. At alkaline pH (9–10), a rapid decline in water flux (> 50 %) and higher membrane rejection was observed, as a consequence of the increased electrostatic repulsion forces between the membrane surface and organic species. Parallel factor analysis (PARAFAC) modeling and size exclusion chromatography (SEC) provides detailed insights into the WEOM compositional behavior at different pH levels. Ozonation at higher pH significantly reduced the apparent molecular weight (MW) of WEOM in the 4000–7000 Da range by transforming the large MW (humic-like) substances into small hydrophilic fractions. Fluorescence components C1 (humic-like) and C2 (fulvic-like) exhibited a predominant increase/decrease in concentration for all pH conditions during pre-ozonation and nanofiltration treatment process, however, the C3 (protein-like) component was found highly associated with the reversible and irreversible membrane foulants. The ratio C1/C2 provided a strong correlation with the formation of total trihalomethanes (THMs) (R2 = 0.9277) and total haloacetic acids (HAAs) (R2 = 0.5796). The formation potential of THMs increased, and HAAs decreased, with the increase of feed water pH. Ozonation markedly reduced the formation of THMs by up to 40 % at higher pH levels, but increased the formation of brominated-HAAs by shifting the formation potential of DBPs towards brominated precursors.

Tree mycorrhizal type controls over soil water-extractable organic matter quantity and biodegradation in a subtropical forest of southern China

Tree mycorrhizal type is believed to be a key factor controlling soil organic matter dynamics in forests, yet its effects on soil dissolved organic matter (DOM) quantity and biodegradation remain unknown, especially in sub/tropical forests. Here, we used soil water-extractable organic matter (WEOM) as a proxy for soil DOM, and measured water-extractable organic carbon (WEOC), total nitrogen (WETN), and total phosphorus (WETP) concentrations, and WEOM chemical composition (i.e., aromaticity and molecular weight) in the 0–10 cm and 10–30 cm mineral soil depths along the gradient ranging from arbuscular mycorrhizal (AM)- to ectomycorrhizal (ECM)-associated tree dominance in a subtropical secondary forest of southern China. Subsequently, we assessed soil WEOM biodegradation using a 42-day aerobic incubation experiment. At each soil depth, WEOC concentration and WEOC:WETN ratio increased with increasing ECM tree dominance, whereas WEOM aromaticity and molecular weight exhibited an opposite changing trend. Moreover, WEOC concentration correlated positively with soil organic C, total N, total P, and sand content, but correlated negatively with soil pH and clay and silt contents. Soil WEOM biodegradation did not change with ECM tree dominance in 0–10 cm depth but decreased with increasing ECM tree dominance in 10–30 cm depth. Soil WEOM biodegradation was negatively related to WEOC:WETP and WETN:WETP ratios, but was positively related to WEOM aromaticity and molecular weight. These findings suggested that tree mycorrhizal type is a potential indicator of soil DOM concentration and biodegradation, and highlight that the shifts in dominance of AM- and ECM-associated trees will influence soil DOM dynamics by altering soil texture, C:N:P stoichiometry, and C quality in subtropical forests.

Biological Filtration is Resilient to Wildfire Ash-Associated Organic Carbon Threats to Drinking Water Treatment.

Elevated/altered levels of dissolved organic matter (DOM) in water can be challenging to treat after wildfire. Biologically mediated treatment removes some DOM; here, its ability to remove elevated/altered postfire dissolved organic carbon (DOC) resulting from wildfire ash was investigated for the first time. Treatment of wildfire ash-amended (low, moderate, high) source waters by bench-scale biofilters was evaluated in duplicate. Turbidity and DOC were typically well-removed (effluent turbidity ≤0.3 NTU; average DOC removal ∼20%) in all biofilters during periods of stable source water quality. Daily DOC removal across all biofilters (ash-amended and controls) was generally consistent, suggesting that (i) the biofilter DOC biodegradation capacity was not deleteriously impacted by the ash and (ii) the biofilters buffered the ash-associated increases in water extractable organic matter. DOM fractionation indicates this was because the biodegradable low molecular weight neutral fractions of DOM, which increased with ash addition, were reduced by biofiltration while humic substances were largely recalcitrant. Thus, biological filtration was resilient to wildfire ash-associated DOM threats to drinking water treatment, but operational resilience may be compromised if the balance between readily removed and recalcitrant fractions of DOM change, as was observed during brief periods herein.

Effect of cover cropping on soil organic matter characteristics: Insights from a five-year field experiment in Nebraska

Cover crops are increasingly incorporated into crop rotations to improve soil health including the postulated positive impact on soil organic carbon (SOC) storage. However, a consistent positive response of SOC to cover cropping was not observed in several studies, necessitating further clarification of mechanisms by which cover crops impact soil organic matter (SOM) dynamics. In this study, we evaluated the effect of five years of winter cover cropping within a corn-soybean rotation in Nebraska under no-till on SOC and nitrogen (N) storage in the bulk topsoil (0–10 cm depth) as well as in particulate (free and occluded), water-extractable, and mineral-associated organic matter fractions. Furthermore, cover crop effects on potential C and N losses from the topsoil in form of CO2 and N2O emissions and leached organic C and N were evaluated. The different SOM fractions were analyzed for their functional group composition using diffused reflectance infrared Fourier transform spectroscopy to account for potential changes in the SOM composition due to cover crops. Our results indicated no effect of cover cropping on C and N losses (gaseous and leaching) and on bulk SOC and total N storage five years after implementation. Differences were detected for the SOM fractions, with a positive effect of cover crops on the amount of water-extractable organic matter. The spectroscopic results suggested a significant cover crop effect on the composition of free particulate organic matter fraction. Both fractions seem to be suitable for detecting early changes in SOM characteristics due to cover cropping under the studied conditions. A significantly measurable impact on bulk SOC and N storage in 0–10 cm depth for the studied corn-soybean systems might require more than five years of cover cropping, or management changes that target an increase in cover crop biomass production and input into the soil.

SOIL RESPIRATION AS MICROBIAL RESPONSE TO THE ENDOGEN INPUT OF BIO-SYNTHESIZED ORGANIC MATTER AND ITS IMPLICATION IN CARBON SEQUESTRATION

Active C, as a measure of the level of chemical oxidation of organic matter, reflects the carbon available to microorganisms. Soil respiration, as a direct way to estimate edaphic microbial activity, could be a measure of the fluctuations of carbon stocks in soils. To determine the ability of soil respiration to evaluate such fluctuations we used soils with an increased content of organic carbon, constant optimal conditions, to eliminate the disturbing factors, analyzed in a short period of time. The influence of the specific decomposition rates of these soils was assessed by both spot determinations of soil respiration and analyzing the flux of CO2 from ex situ soil samples, under standard experimental conditions, to highlight carbon storage in such soils. Reference data can be accumulated through the analysis of these parameters, which compared with the results of quantitative/qualitative determinations regarding the changes in the content of microbial biomass, the content of fulvic sub-fractions, the fluorescence of dissolved organic material and the evolution of the content of siderophores, could be considered, by their own evolutions, as arguments in sustaining the use of respiration in the efficient estimation of carbon storage evolution in the soils. The analyses of these parameters were carried out in two phases, for comparing initial and final data of experiment (after 30 days). The soils had different levels of the respiration potential between phases. The level of soil respiration was reduced in time between 4.27-14.60%, in each soil. The CO2 flux showed, in time, a continuous decreasing trend in both soils. In the case of Mollic Histic Gleysol (Salinic), the coefficient of determination has the value R2=0.92 for the flux determined in the final phase. The levels of microbial biomass of both soils were increased significantly at the end of the experiment. In the case of Mollic Histic Gleysol (Salinic), microbial biomass increased from 456±23.12 μgC∙g-1 to 514±24.57 μgC∙g-1 soil. The fulvic sub-fractions A-D of both soils revealed significant accumulates of soluble organic compounds, with different molecular weights and complexity levels, after 30 days of incubations in standard conditions. The fluorescent components present in the water-extractable organic matter were highlighted by imagistic method. The highest degree of storages of newly bio-synthesized compounds of carbon was registered in organic matter of Mollic Histic Gleysol (Salinic). The intensity of siderophores biosynthesis increased over time, starting from an initial lower presence in the Mollic Gleysol (Salinic) (with Ø 11 mm halo), which were followed by an increasing of siderophores content and availability of iron, at the end of the experimental period. Accumulations of siderophores in the Mollic Histic Gleysol (Salinic) determined a Ø 31 mm halo diameter.

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.