Leaching and microbial degradation of dissolved organic matter from salt marsh plants and seagrasses

Dissolved organic matter (DOM) is outwelled from highly productive salt marshes, but its sources and fates are unclear. To examine common salt marsh plants as sources of coastal DOM, two dominant salt marsh vascular plants Spartina alterniflora and Juncus roemarianus, and two major coastal seagrasses Syringodium filiforme and Halodule wrightii, were collected from a Florida salt marsh and studied using laboratory incubation experiments. We investigated the leaching dynamics of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and chromophoric dissolved organic matter (CDOM) from these plants, in conjunction with our field investigations of the sources and outwelling of DOM from Florida salt marshes. The leaching of DOM and CDOM from the plants was a rapid process, and leaching rates were 65–288 µM/g dry weight/day for DOC and 3.8–16 µM/g dry weight/day for TDN from different plants in the bacteria-inhibited incubations. DOC was proportional to TDN in the leachates, but the quantity of C and N leached was dependent on the species and growth stage of the plants. At the end of the 25-day experiments, 5.4–23 % and 10–45 % of solid phase C and N were released into DOC and TDN pools, respectively. Bacteria played an important role during the leaching process. The majority of DOC and TDN leached from marsh plants and seagrasses was labile and highly biodegradable with 56–90 % of the leached DOC and 44–72 % of the leached TDN being decomposed at the end of the experiments. The fluorescence measurements of CDOM indicate that organic matter leached from marsh plants and seagrasses contained mainly protein-like DOM which was degraded rapidly by bacteria. Our study suggests that leaching of DOM from salt marsh plants and seagrasses provide not only major sources of DOC, TDN, and CDOM that affect many biogeochemical processes, but also as important food sources to microbial communities in the marsh and adjacent coastal waters.     

Allochthonous subsidies of organic matter across a lake–river–fjord landscape in the Chilean Patagonia: Implications for marine zooplankton in inner fjord areas

Ecosystems can act as both sources and sinks of allochthonous nutrients and organic matter. In this sense, fjord ecosystems are a typical interface and buffer zone between freshwater systems, glaciated continents, and the coastal ocean. In order to evaluate the potential sources and composition of organic matter across fjord ecosystems, we characterized particulate organic matter along a lake–river–fjord corridor in the Chilean Patagonia using stable isotope (δ¹³C) and lipid (fatty acid composition) biomarker analyses. Furthermore, estimates of zooplankton carbon ingestion rates and measurements of δ¹³C and δ¹⁵N in zooplankton (copepods) were used to evaluate the implications of allochthonous subsidies for copepods inhabiting inner fjord areas. Our results showed that riverine freshwater flows contributed an important amount of dissolved silicon but, scarce nitrate and phosphate to the brackish surface layer of the fjord ecosystem. Isotopic signatures of particulate organic matter from lakes and rivers were distinct from their counterparts in oceanic influenced stations. Terrestrial allochthonous sources could support around 68–86% of the particulate organic carbon in the river plume and glacier melting areas, whereas fatty acid concentrations were maximal in the surface waters of the Pascua and Baker river plumes. Estimates of carbon ingestion rates and δ¹³C in copepods from the river plume areas indicated that terrestrial carbon could account for a significant percentage of the copepod body carbon (20–50%) during periods of food limitation. Particulate organic matter from the Pascua River showed a greater allochthonous contribution of terrigenous/vascular plant sources. Rivers may provide fjord ecosystems with allochthonous contributions from different sources because of the distinct vegetation coverage and land use along each river’s watershed. These observations have significant implications for the management of local riverine areas in the context of any human project that may modify terrestrial habitats as well as the productivity, food webs, and community structure of rivers, lakes, fjords, and the coastal ocean in the Chilean Patagonia.     

Export and degassing of terrestrial carbon through watercourses draining a temperate podzolized catchment

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO₂ across the water-atmosphere interface were estimated to assess the relative importance of CO₂ evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8.4 ± 3.4 and 1.6 ± 0.6 mg L^?1, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1.3 ± 1.4 μg L^?1) and the relatively constant δ¹³C-POC value (near ?28 ‰) throughout the year reveal this POC originates from terrestrial C₃ plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6.6 ± 2.2 mg L^?1). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5.9 ± 2.2 mg L^?1). The δ¹³C-DIC value around ?20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO₂ between 1,000 and 10,000 ppmv, all watercourses were a source of CO₂ to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year^?1 or 6 t C km^?2 year^?1, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO₂ degassing to the atmosphere (34 %).     

Land–ocean distribution of allochthonous organic matter in surface sediments of the Chiloé and Aysén interior seas (Chilean Northern Patagonia)

We investigated the relative distribution of allochthonous (i.e., terrigenous) organic matter in the complex, continuous, river–fjord–sound–channel–gulf system of Chile’s North Patagonia (41.5–46.5°S) in order to establish whether this organic matter can reach the open ocean or whether it is largely retained near its fluvial sources. Grain size distribution, total organic carbon and total nitrogen contents, and carbon stable isotope contents (δ¹³C) were quantified in 53 surface sediment samples collected during the CIMAR Fiordos cruises 1, 4, 8, and 10, as were salinity and silicic acid concentrations in the surface waters. A principal component analysis segregated the Chiloé and Aysén interior seas into two zones: (i) the continental fjords, with sediment enriched in allochthonous organic matter, having higher C:N molar ratios (10–14) and lower δ¹³C composition (?23‰ to ?27‰); and (ii) the channels and gulfs, with a prevalent autochthonous marine source, having lower C:N values (6–10) and higher δ¹³C composition (?20‰ to ?23‰). Estuarine waters with low salinity (2–30) and high silicic acid (10–90 μM) were associated with high C:N ratios and low δ¹³C in surface sediments, meaning that terrestrial organic matter was transported up to the mouth of the continental fjords. A two-source mixing model confirmed that allochthonous (terrestrial) organic matter contents (50–90%) associated with local river discharges were present within the continental fjords. On the contrary, autochthonous (marine) organic matter was prevalent (50–90%) at the sites in the marine influenced channels, sounds, and gulfs.     

Origin, enzymatic response and fate of dissolved organic matter during flood and non-flood conditions in a river-floodplain system of the Danube (Austria)

Spectroscopic techniques and extracellular enzyme activity measurements were combined with assessments of bacterial secondary production (BSP) to elucidate flood-pulse-linked differences in carbon (C) sources and related microbial processes in a river-floodplain system near Vienna (Austria). Surface connection with the main channel significantly influenced the quantity and quality of dissolved organic matter (DOM) in floodplain backwaters. The highest values of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) were observed during the peak of the flood, when DOC increased from 1.36 to 4.37 mg l^?1 and CDOM from 2.94 to 14.32 m^?1. The flood introduced DOC which consisted of more allochthonously-derived, aromatic compounds. Bacterial enzymatic activity, as a proxy to track the response to changes in DOM, indicated elevated utilization of imported allochthonous material. Based on the enzyme measurements, new parameters were calculated: metabolic effort and enzymatic indices (EEA 1 and EEA 2). During connection, bacterial glucosidase and protease activity were dominant, whereas during disconnected phases a switch to lignin degradation (phenol oxidase) occurred. The enzymatic activity analysis revealed that flooding mobilized reactive DOM, which then supported bacterial metabolism. No significant differences in overall BSP between the two phases were detected, indicating that heterogeneous sources of C sufficiently support BSP. The study demonstrates that floods are important for delivering DOM, which, despite its allochthonous origin, is reactive and can be effectively utilized by aquatic bacteria in this river-floodplain systems. The presence of active floodplains, characterized by hydrological connectivity with the main channel, creates the opportunity to process allochthonous DOC. This has potential consequences for carbon flux, enhancing C sequestration and mineralization processes in this river-floodplain system.     

Seasonal and inter-annual variability of water quality in the Uruguay River,Argentina

Abstract

Water quality of the Uruguay River was evaluated with multi-parametric (temperature, turbidity, conductivity, pH, dissolved oxygen) and sediment trap data (particle flux, total organic carbon and nitrogen contents) and correlated to precipitation, river discharge and El Niño Southern Oscillation (ENSO) indices for the period 2006–2011. Hydro-meteorological parameters averaged 24–85% variability with coincident precipitation (200–400 mm month^-1), discharge (7000–28 000 m³ s^-1) and turbidity peaks (50–80 NTU) in the austral spring, and absolute maxima during the El Niño 2009 episode. Spectral analysis of discharge and sea-surface temperature anomaly (SSTA) showed consistent variance maxima at approx. 3 and 1.5 years. Deseasonalized discharge was significantly correlated to SSTA. During river floods, pH decreased (from 7.5 to 6.6) and particle dynamics peaked (turbidity: 15–80 NTU; vertical fluxes: 20–200 g m^-2 d^-1; total solid load: <1000 to 100 000 t d^-1),whereas TOC remained stable (3.2 ± 0.8%) and C/N ratios increased (10–12) due to the higher contribution of terrestrial detritus.     

EXPORTS OF ORGANIC CARBON IN BRITISH RIVERS

This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7–103·5 kg ha⁻¹ year⁻¹, with a median flux of 31·9 kg ha⁻¹ year⁻¹ in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two–thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change. © 1997 by John Wiley & Sons, Ltd.     

Variation in the photochemical lability of dissolved organic matter in a large boreal watershed

Boreal watersheds contain a vast quantity of terrestrially derived dissolved organic matter (DOM) originating from wetland and forest soils, yet variation in the potential for photochemical transformation of boreal aquatic DOM sources remains poorly understood. Laboratory solar radiation exposure experiments were conducted on DOM samples collected in three seasons, across nine sites, representing contrasting catchment composition and watershed position to assess variation in the photochemical lability of boreal DOM source and stable carbon isotopic signature (δ¹³C) of photomineralized DOM. Dissolved organic carbon (DOC) loss rates during laboratory exposure were lowest in summer, suggesting that DOM may have been more photo-degraded during summer. DOM from upstream portions of forested stream sites and wetland-influenced sites was more photolabile relative to downstream portions and the river DOM, suggesting potential losses in photolabile DOM downstream and in the lower reaches of the watershed. Increased a₂₅₄:a₃₅₀ and spectral slope following sample exposure suggest photoproduction of low molecular weight (LMW) CDOM and/or a higher photoreactivity of high molecular weight versus LMW compounds. Photomineralization of nitrogen was regulated by organic nitrogen concentration and resulted in NH₄ ⁺-photoproduction rates between 0.01 and 0.3?μM N?h^?1 and ecologically significant increases in NH₄ ⁺ for these waters. The δ¹³C of the photomineralized DOM was positively correlated to initial DOC concentration and generally lower when initial DOC concentrations were lower, suggesting variation in photomineralized DOM δ¹³C may be a result of kinetic isotope fractionation. Results from this study demonstrate significant variation in the photochemical lability of boreal watershed sources of DOM. Such variation suggests landscape and environmental change has the potential to alter the biogeochemical role photochemical transformations play in downstream portions of boreal watersheds.     

Sources and distribution of organic matter in northern Patagonia fjords,Chile (∼44–47°S): A multi-tracer approach for carbon cycling assessment

We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (～44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ¹³C, δ¹⁵N), and biomarker (n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ¹³C, and δ¹⁵N from organic matter were 0.127±0.010, ?19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, ?29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ～4280 km², suggest that carbon accumulation may account for between 2.3 and 7.8×10⁴ ton C yr^?1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×10⁴ ton yr^?1, which corresponds to an assimilation rate of CO₂ by marine photosynthesis between 0.06 and 0.23×10⁶ ton yr^?1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ～240,000 km², may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO₂.     

Determination of dissolved organic matter in streamwater using visible spectrophotometry

Absorbance at 360 nm was measured on 44 filtered streamwater samples of different dissolved organic matter (DOM) contents. A regression equation of DOM on absorbance predicted DOM with a standard error of estimate of 1.26 mgl^?1, Use of a published equation relating dissolved organic carbon (DOC) to absorbance gave DOC values for the samples which were consistent with measured DOM. The method offers considerable potential for rapid quantification of dissolved organic matter concentrations in streamwater.     

Photochemical degradation of dissolved organic matter from streams in the western Lake Superior watershed

The input and fate of dissolved organic matter (DOM) can have important consequences for coastal zone productivity in large lakes and oceans. Chromophoric DOM (CDOM) is often delivered to coastal zones from rivers and streams and affects light penetration in a water column. CDOM can protect biota from damaging ultraviolet (UV) light by acting as sunscreen, resulting in increased ecosystem productivity. Alternatively, CDOM can decrease ecosystem productivity by absorbing light needed for photosynthesis and forming photoreaction products that are harmful to coastal zone biota. Increased urbanization of watersheds and seasonal differences in weather patterns change the delivery pathways, reactivity, input, and energy flow of DOM (and its CDOM component) into aquatic systems. This study investigated the effects of watershed and season on the concentrations and potential photodegradation of stream-derived DOM in Lake Superior tributaries, chosen to be geographically and geologically similar but differing in land use. Organic carbon analysis, UV–Visible spectrophotometry, and terrestrial (land use) analysis were used to investigate differences among samples and sample treatments. The major differences in DOM concentration and photochemical response appeared seasonal rather than site specific, with snow-melt samples showing stronger and more consistent changes in UV–Visible parameters while base-flow samples showed stronger and more consistent losses in DOC.     

Processing of humic-rich riverine dissolved organic matter by estuarine bacteria: effects of predegradation and inorganic nutrients

The bioavailability of predegraded dissolved organic matter (DOM) from a humic-rich, boreal river to estuarine bacteria from the Baltic Sea was studied in 39-day bioassays. The river waters had been exposed to various degrees of bacterial degradation by storing them between 0 and 465 days in dark prior to the bioassay. The resulting predegraded DOM was inoculated with estuarine bacteria and the subsequent changes in DOM quantity and quality measured. During the incubations, dissolved organic carbon (DOC) and oxygen concentrations decreased, indicating heterotrophic activity. Coloured DOM was degraded less than DOC, indicating a selective utilization of DOM, and humic-like fluorescence components increased during the incubations. The amount of DOC degraded was not affected by the length of DOM predegradation. The percentage of bioavailable DOC (%BDOC) was higher in experiment units with added inorganic nitrogen and phosphorus than without addition (on average 13.5 % and 9.0, respectively), but had no effect on the degradation of fresh, non-predegraded, DOC (%BDOC 12.0 %). Bacterial growth efficiency (BGE) was highest (65 ± 2 %) in the units with fresh DOM, and lowest in units with predegraded DOM and no added inorganic nutrients (11 ± 4 %). The addition of inorganic nutrients increased the BGE of predegraded DOM units by an average of 28 ± 4 %. There was no significant effect on BGE by length of predegradation after the initial drop (<3 months). This study suggests that both the length of predegradation and the inorganic nutrient status in the receiving estuary has consequences to carbon cycling and will determine the amount of terrestrial-derived DOC being ultimately assimilated into marine food webs.     

Comparisons of wetland and drainage lake influences on stream dissolved carbon concentrations and yields in a north temperate lake-rich region

Processes occurring at various scales interact to influence the export of organic carbon from watersheds to freshwater ecosystems and eventually the ocean. The goal of this study was to determine if and how differences in wetland extent and presence of lakes influenced dissolved organic carbon (DOC) concentrations and yields in streams. We monitored stream flow, DOC and dissolved inorganic carbon concentrations periodically for 2 years at four sites with forested watersheds, four sites with wetland watersheds, and four sites with wetland watersheds that also contained in-network lakes. As expected, the presence of wetlands resulted in higher DOC concentrations and yields, but the impact of lakes was less clear on the magnitude of DOC concentrations and yields. With respect to temporal dynamics, we found positive relationships between stream flow and DOC concentration (median r² = 0.89) in streams without upstream lakes. The relationships for forested sites are among the strongest reported in the literature, and suggest a clear shift in hydrologic flowpath from intersecting mineral soils at low flow, to organic soils at high flow. In streams with upstream lakes, the relationship between flow and concentration was non-significant for three of four sites unless time lags with flow were applied to the concentration data, after which the relationship was similar to the non-lake streams (median r² = 0.95). These findings suggest that lakes buffering temporal patterns in streams by hydrologically delaying pulses of carbon, but provide little support that in-line lakes have a net effect on carbon exports in this region.     

Dissolved Organic Carbon Fractions Formed during Composting of Municipal Solid Waste: Properties and Significance

The properties and transformation of dissolved organic matter (DOM) extracted (10 L water per kilogram compost) from municipal solid waste (MSW) compost at five stages (days 47, 77, 105, 126, and 187) of composting were investigated. The DOM was fractionated into hydrophobic or hydrophilic neutrals, acids, and bases. The unfractionated DOM, the hydrophobic acids and neutrals (HoA and HoN, respectively), and the hydrophilic neutrals (HiN) fractions were studied using solid-state ¹³C-NMR, FTIR, and DRIFT spectroscopy. The HoA fraction was found to be the dominant (percentage of total DOM) hydrophobic fraction, exhibiting a moderate increase during composting. The HoN fraction increased sharply from less than 1% to 18% of the total DOM during 187 days of composting, while the hydrophobic bases (HoB) exhibited the opposite trend. The HiN represented the major fraction of the hydrophiles up to 120 days of composting, decreasing thereafter by 38%. The relative concentration of the hydrophilic acids and bases (HiA and HiB, respectively) exhibited no consistent trend during composting. DRIFT spectra of the unfractionated DOM taken from the composting MSW revealed a decreasing level of polysaccharide structures with time. The ¹³C-NMR and FTIR spectra of the HoA fraction exhibited a polyphenol-humic structure, whereas the HoN spectra exhibited strong aliphatic features. The spectra of the HiN fraction confirmed its polysaccharide nature. During the final stage of composting, the DOM concentration was steady, while a relative decrease of HiN concomitant with an increase of HoA and HoN fractions was observed. These indicate that the DOM contained a low concentration of biodegradable organic matter and a higher content of macromolecules related to humic substances. The biological significance and heavy metal binding of these fractions are being studied based on earlier observations showing enhanced plant growth in the presence of DOM extracted from mature as opposed to immature compost.     

The freshwater dissolved organic matter fluorescence–total organic carbon relationship

The fluorescent properties of dissolved organic matter (DOM) enable comparisons of humic‐like (H‐L) and fulvic‐like (F‐L) fluorescence intensities with dissolved organic carbon (DOC) in aquatic systems. The fluorescence‐DOC relationship differed in gradient, i.e. the fluorescence per gram of carbon, and in the strength of the correlation coefficient. We compare the fluorescence intensity of the F‐L and H‐L fractions and DOC of freshwater DOM in north Shropshire, England, featuring a river, wetland, spring, pond and sewage DOM sources. Correlations between fluorescence and DOC varied between sample sites. Wetland water samples for the F‐L peak gave the best correlation, r = 0·756; the lowest correlation was from final treated sewage effluent, r = 0·167. The relationship between fluorescence and DOC of commercially available International Humic Substances Society standards were also examined and they generally showed a lower fluorescence per gram of carbon for the F‐L peak than the natural samples, whereas peat wetland DOM gave a greater fluorescence per gram of carbon than river DOM. Here, we propose the strength of the fluorescence–DOC correlation to be a useful tool when discriminating sources of DOM in fresh water. Copyright © 2007 John Wiley & Sons, Ltd.     

Compositions and constituents of freshwater dissolved organic matter isolated by reverse osmosis

Dissolved organic matter (DOM) from riverine and lacustrine water was isolated using a reverse osmosis (RO) system. Solid-state ¹³C nuclear magnetic resonance (¹³C NMR) was used to quantitatively evaluate the compositions and constituents of DOM, which are compared with previous investigations on marine DOM. Results indicated that concentration factor (CF) was a key metric controlling yield and sorption of DOM on the RO system. The sorption was likely non-selective, based on the ¹³C NMR and δ¹³C analyses. Carbohydrates and lipids accounted for 25.0–41.5% and 30.2–46.3% of the identifiable DOM, followed by proteins (18.2–19.8%) and lignin (7.17–12.8%). The freshwater DOM contained much higher alkyl and aromatic C but lower alkoxyl and carboxyl C than marine DOM. The structural difference was not completely accounted for by using structure of high molecular weight (HMW) DOM, suggesting a size change involved in transformations of DOM during the transport from rivers to oceans.     

Spectroscopic Characterization of Organic Structures and Organic‐Inorganic Interactions in Paper Mill Sludge

The organic composition and organic‐inorganic interaction in paper mill sludge (PS) solvent extracts (hexane, ethyl acetate, acetone and ethanol) and humic fractions, humic acid (HA) and humin (HU) were studied by electron paramagnetic resonance spectroscopy (EPR), proton and carbon‐13 nuclear magnetic resonance spectroscopy (¹H NMR; ¹³C NMR), Fourier‐transformed infrared spectroscopy (FTIR), and ultraviolet‐visible spectroscopy (UV‐vis). The strategy of fractionating the PS, sequentially, with organic solvents of increasing polarity is a reliable analytical procedure for humic substance sample separation because it results in more purified fractions. FTIR, ¹H NMR and ¹³C NMR results showed that hexane extract consisted mainly of aliphatic hydrocarbon structures. Their contents in the extracts decreased as the polarity of the extracting solvent increased and the content of oxygen functional groups increased. Carboxylic and carboxylate functional groups were found in the acetone extract, and ester and ether functions were predominantly found in the ethanol extract. EPR spectra revealed some Fe³⁺ complexes with rhombic structure (g₁ = 4.3; g₂ = 9.0) in the humic fractions and in all solvent extracts, except hexane. Quasi‐octahedral Fe³⁺ complexes (g = 2.3; ΔH_pp ≤ 400 G) were found in the HU fraction and in the acetone extract. The organic free radical content in the HA fraction was higher than the non‐fractionated PS sample and HU fraction.