Fulvic Components Research Articles

Differences in dissolved organic matter (DOM) composition of soils from native eucalypt forests and exotic pine plantations impacted by wildfire in Southwest Australia

In recent decades, plantations of exotic species have transformed large tracts of forested landscapes, where they often form a mosaic with the surrounding native forest. Wildfires have also increased in frequency and intensity globally, impacting both plantation and native forests and affecting soil organic matter (SOM), including dissolved organic matter (DOM). DOM of soils is considered highly labile and likely sensitive to effects of fire and changes in land use. However, it is unclear how wildfire influences key characteristics of DOM in exotic plantations compared to native forest. In this study, we investigated the impacts of wildfire on chemical properties of DOM in exotic Pinus radiata plantations and adjacent native jarrah (Eucalyptus marginata) forest in southwest Australia. We coupled fluorescence excitation-emission matrix parallel factor analysis (EEM-PARAFAC) and 1H‐NMR with soil nutrient analyses to characterise soil DOM (0–5 cm) from burned and unburned plots of each forest type, four months after the fire. Four fluorescent components (humic-, fulvic-, microbial and protein-like substances) were derived from soil DOM using the PARAFAC model. DOM from burnt forest soils was predominantly of lower molecular weight (E2:E3) and contained a greater microbial-derived component and a lower fulvic component than unburnt forest. As expected, soil pH and labile inorganic P increased in both forest types after fire. By contrast, an increase in the amount of DOC after fire was only recorded in native forest, while δ15N signatures were more enriched in pine plantation soils after fire but not in native forest soils. Differences in impacts of fire on DOC and δ15N between forest types suggest a different response to fire according forest type, even though these differences were not evident above ground. The 1H NMR spectra of DOM revealed all samples were largely dominated by aliphatic structures. However, there were no significant differences in the overall spectra between forest types or between burnt and unburnt forests. Overall, our findings suggest that transformations in soil DOM after fire were affected by feedbacks between forest type and fire (where soils from pine plantations appeared to be more affected by fire).

The impact of combining data sets of fluorescence excitation - emission matrices of dissolved organic matter from various aquatic sources on the information retrieved by PARAFAC modeling

Despite that fluorescence spectroscopy coupled with Parallel Factor Analysis (PARAFAC) has been widely used in the investigation of Fluorescent Dissolved Organic Matter (FDOM) in aquatic systems, the proper performance of PARAFAC analysis on datasets originating from various sources is not to be taken for granted. In this study, we examine the impact of the co-analysis of datasets from various natural water systems located in the same geographical region in the Eastern Mediterranean Sea. For this purpose three datasets were formed representative of open sea waters (SW), rivers and streams (RV) and lagoons (LG). The Excitation Emission Matrices (EEMs) derived from fluorescence analysis were subjected to individual PARAFAC analysis per dataset as well as combined analyses i.e.: SWRV, SWLG, RVLG, ALL (SW-RV-LG). We evaluated the reliability of the components that were validated in the combined models through the investigation of model’s residuals and components correlation. We also assessed the similarity of the common identified components among models in regards of: (a) spectral position, by calculating the Tucker congruence coefficient (TCC) of the excitation and emission loadings of the PARAFAC components, and (b) fluorescence intensity, through regression analysis of Fmax, among models. Our analysis showed that for natural waters of various sources within the same geographical region, combined PARAFAC modeling can have both negative as well as positive impact. In the case of the combined SWLG and RVLG models, the PARAFAC analysis was able to resolve the fulvic component that was initially observed only in the LG dataset and thus a new component for SW and RV datasets was resolved. The fulvic-like component was actually identified for the first time in the open sea using the combined datasets. Moreover in the combined SWRV analysis tyrosine-like component was resolved which was found initially only in the RV dataset. Contrary, tyrosine-like component was lost in the combined RVLG dataset. We also show that the resolution of extra components in a combined analysis is not always a good fit for the dataset and the model should be assessed in terms of residuals prior acceptance. Finally, our study proposes that the similarity of the common components between combined and individual models is largely dependent on the similarity between the components of the individual models and that the estimation of the Fmax of a component is probably less affected by data diversity compared to the estimation of its spectral position.

A Mechanistic Exploration of Natural Organic Matter Aggregation and Surface Complexation in Smectite Mesopores

Soil minerals and organic matter play critical roles in nutrient cycling and other life-essential biogeochemical processes, yet the structural and dynamical details of natural organic matter (NOM) film formation on smectites are not fully understood on the molecular scale. XRD of Suwannee River NOM-hectorite (a smectite clay) complexes shows that the humic and fulvic components of NOM bind predominantly at the external surfaces of packets of smectite platelets rather than in the interlayer slit pores, suggesting that the key behavior governing smectite-NOM interactions takes place in mesopores between smectite particles. New molecular dynamics modeling of a ∼110 Å H2O-saturated smectite mesopore at near-neutral pH shows that model NOM molecules initially form small clusters of 2-3 NOM molecules near the center of the pore fluid. Formation of these clusters is driven by the hydrophobic mechanism, where aromatic/aliphatic regions associate with one another to minimize their interactions with H2O, and charge-balancing cations associated with the deprotonated carboxylate sites are located only at the outer surface of these clusters. Despite hydrophobicity driving the initial clustering, NOM clusters are formed more quickly when high-charge-density cations like Ca2+ are present vs low-charge-density cations like Cs+, as the former cations more effectively minimize the electrostatic repulsions between the negatively charged NOM molecules. Once the small hydrophobicity-driven NOM clusters form, the simulations show that Ca2+ promotes the aggregation of NOM clusters through tetradentate Ca2+ bridges involving carboxylate groups on two different NOM clusters. Importantly, our studies indicate that Ca2+ plays a crucial role in binding the NOM clusters to the smectite surface, which occurs through multiple quaternary complexes (Ob)-H2O-Ca2+-COO-NOM. In contrast, Cs+ never forms any coordination or acts like bridges between NOM molecules nor as ion bridges to the smectite surface. Additionally, we observe the formation of a metastable superaggregate involving all 16 NOM molecules several times in a Ca2+-bearing mesopore fluid. Superaggregates are never observed in the simulations involving Cs+. The modeling results are fully consistent with helium ion microscope images of NOM-hectorite complexes suggesting that NOM surface films develop when preformed NOM clusters interact with smectite surfaces. Overall, the binding of NOM clusters to the outer surfaces of smectite particles and the formation of large NOM aggregates at neutral pH occur through cation bridging, and cation bridging only occurs when high-charge-density cations like Ca2+ are present.

Gravimetric Quantification of Hydrophobic Fulvic Acids in Lignite Material Using Acetone

Abstract This study evaluated the gravimetric quantification of fulvic components in lignite material. The current standard method considered only components soluble in both alkali and acid solutions and adsorpted to a hydrophobic resin (DAX-8) at pH 1, identified as hydrophobic fulvic acids (HFA). HFA were desorpted using 0.1 M sodium hydroxide (NaOH) and run through a cation exchange resin (IR-120) to remove Na ions. HFA were quantified gravimetrically, including ash corrections. Steps involving IR-120, however, resulted in a large volume of analytical solution and a significant time of analysis, which became a disadvantage of this method. A modification to this method was proposed by replacing NaOH with acetone to desorpt HFA from DAX-8. IR-120 was not required, and a smaller volume of analytical solution was produced. Gravimetric, Fourier transform infrared (FTIR), and ionic analyses were completed in this study. Both standard and modified methods resulted in the presence of hydrophobic neutrals within the quantification of HFA. Acetone at 50% strength produced HFA results corresponding to those of NaOH. An adequate regeneration of DAX-8 was achieved using the modified method but not with the standard method. The modified method produced higher ash contents, but they did not affect its accuracy. This method significantly reduced the overall analytical time. This study showed the potential of acetone as a desorpting agent in the gravimetric quantification of HFA in lignite material.

Gravimetric Quantification of Hydrophobic Fulvic Acids in Lignite Material Using Acetone.

This study evaluated the gravimetric quantification of fulvic components in lignite material. The current standard method considered only components soluble in both alkali and acid solutions and adsorpted to a hydrophobic resin (DAX-8) at pH 1, identified as hydrophobic fulvic acids (HFA). HFA were desorpted using 0.1 M sodium hydroxide (NaOH) and run through a cation exchange resin (IR-120) to remove Na ions. HFA were quantified gravimetrically, including ash corrections. Steps involving IR-120, however, resulted in a large volume of analytical solution and a significant time of analysis, which became a disadvantage of this method. A modification to this method was proposed by replacing NaOH with acetone to desorpt HFA from DAX-8. IR-120 was not required, and a smaller volume of analytical solution was produced. Gravimetric, Fourier transform infrared (FTIR), and ionic analyses were completed in this study. Both standard and modified methods resulted in the presence of hydrophobic neutrals within the quantification of HFA. Acetone at 50% strength produced HFA results corresponding to those of NaOH. An adequate regeneration of DAX-8 was achieved using the modified method but not with the standard method. The modified method produced higher ash contents, but they did not affect its accuracy. This method significantly reduced the overall analytical time. This study showed the potential of acetone as a desorpting agent in the gravimetric quantification of HFA in lignite material.

Electrical Conductivity Breakthrough Experiment and Immobile Water Estimation in Organic Substrates: Is R = 1 a Realistic Assumption?

Core Ideas Breakthrough of NaCl in peat shows that R may differ from 1. Immobile water in peat is difficult to estimate because of NaCl sorption. Solute sorption in peat varies because of uncoiling of organic components. Measuring electrical conductivity (EC) in outflow breakthrough curves (BTCs) is an approach commonly used to define transport parameters because it is rapid, inexpensive, and reliable. Earlier work has raised questions regarding the assumption that the retardation factor may be set at R = 1 when using EC as a tracer in organic soils. This study investigates the breakthrough of a NaCl tracer in a peat–sawdust mixture leached with distilled water. The results show that in such system, R may significantly differ from 1 because of sorption of Na+ and Cl− by peat with the R value estimated by batch experiments to reach 1.9 for Na+ and 1.5 for Cl−. The sorption properties may vary during solute transport because of uncoiling of humic and fulvic components with decreasing ionic strength of the solution.

Optical properties of estuarine dissolved organic matter isolated using cross-flow ultrafiltration

Dissolved organic matter (DOM) from freshwater, mid-salinity, and seawater endmember samples in the Jiulong River Estuary, China were fractionated using cross-flow ultrafiltration with a 10-kDa membrane. The colloidal organic matter (COM; 10 kDa–0.22 μm) retentate, low molecular weight (LMW) DOM (<10 kDa) permeate, and bulk samples were analyzed using absorption spectroscopy and three-dimensional fluorescence excitation-emission-matrix spectroscopy. The UV-visible spectra of COM were very similar to those obtained for permeate and bulk samples, decreasing monotonically with increasing wavelength. Most of the chromophoric DOM (CDOM, expressed as the absorption coefficient a 355 ) occurred in the LMW fraction, while the percentage of CDOM in the colloidal fraction was substantially higher in the freshwater endmember (13.4% of the total) than in the seawater endmember (6.8%). The bulk CDOM showed a conservative mixing behavior in the estuary, while there was removal of the COM fraction and a concurrent addition of the permeate fraction in the mid-salinity sample, implying that part of the colloidal CDOM was transformed into LMW CDOM. Two humic-like components (C1: <250, 325/402 nm; and C2: 265, 360/458 nm) and one protein-like component (C3: 275/334 nm) were identified using parallel factor analysis. The contributions of the C1, C2, and C3 components of the COM fraction to the bulk sample were 2.5%–8.7%, 4.8%–12.6%, and 7.4%–14.7%, respectively, revealing that fluorescent DOM occurred mainly in the LMW fraction in the Jiulong River Estuary. The C1 and C2 components in the retentate and permeate samples showed conservative mixing behavior, but the intensity ratio of C2/C1 was higher in the retentate than in the permeate fractions for all salinity samples, showing that the humic component was more enriched in the COM than the fulvic component. The intensity ratio of C3/(C1+C2) was much higher in the retentate than in the permeate fraction for mid-salinity and seawater samples, revealing that the protein-like component was relatively more enriched in COM than the humic-like component. The contribution of the protein-like component (C3) to the total fluorescence in the retentate increased from 14% in the freshwater endmember to 72% for the seawater endmember samples, clearly indicating the variation of dominance by the humic-like component compared to the protein-like component during the estuarine mixing process of COM.

Variability in dissolved organic matter fluorescence and reduced sulfur concentration in coastal marine and estuarine environments

Abstract Fluorescence characterization of dissolved organic matter (DOM) and measurements of Cr-reducible sulfide (CRS) are presented for 72 coastal marine and estuarine water samples obtained from the USA and Canada. Each sample is identified according to source: terrigenous, autochthonous, wastewater or mixed. Fluorescence data are resolved into contributions from humic, fulvic, tyrosine and tryptophan-like fluorophores. Humic and fulvic-like fluorophores correlate well with dissolved organic C (DOC) ( r 2 = 0.73 and 0.71, respectively) but tyrosine and tryptophan-like fluorophores show no correlation with DOC. Quality factors are identified by normalization of fluorescence contributions to DOC. Humic and fulvic components show no statistical differences between sources but the amino acid-like fluorescence quality factors show significant variations between source, with highest values for autochthonous sources (0.07 ± 0.01 arbitrary fluorescence units per mg of C) versus low values (0.015 ± 0.005) for terrigenous source waters. CRS concentrations are highly variable from 0.07 ± 0.01 to 7703 ± 98 nM and do no correlate with DOC except when terrigenous source waters ( n = 13) are separated out from the total sample set ( r 2 = 0.55). There is an open question in the literature; does DOC source matter in terms of protective effects towards metal toxicity? Here is shown that DOC molecular-level quality does vary and that this variation is mostly in terms of the contributions of amino acids to total fluorescence.

Abiotic interactions of natural dissolved organic matter and carbonate aquifer rock

Abstract Abiotic interactions between natural dissolved organic matter (NDOM) and carbonate aquifer rock may be controlling factors of biogeochemical processes and contaminant fate in carbonate aquifer systems. The importance and effects of these interactions were examined using batch adsorption experiments of soil NDOM and representative carbonate sorbents from the Floridan Aquifer. Adsorption of NDOM carbon to aquifer rocks was well-described using a modified linear model and was mostly reversible. Significant adsorption was observed at higher NDOM concentrations, while the release of indigenous organic matter from the rocks occurred at lower concentrations. Longer interaction periods led to more adsorption, indicating that adsorption equilibrium was not achieved. For relatively pure carbonate rock samples, sorbent surface area was found to be the most important controlling factor of adsorption, whereas the presence of indigenous organic matter and subdominant mineral phases were more important, when they occurred. Preferential adsorption of a high over low molecular weight and humic over fulvic components of NDOM onto carbonate sorbents was detected using liquid size exclusion chromatography and excitation–emission fluorometry, respectively. The presence of NDOM inhibited mineral dissolution, though this inhibition was not proportional to NDOM concentration as surface area and mineralogy of carbonate sorbents played additional roles. Though the NDOM–carbonate rock adsorption mechanism could not be completely determined due to the heterogeneity and complexity of NDOM and sorbent surfaces, it is speculated that both rapid and weak outer-sphere bonding and stronger but slower hydrophobic interaction occur. These results have important implications for groundwater quality and hydrogeologic projects such as aquifer storage and recovery.

Analytical perspective on trace element species of interest in exploration

Analysis of soil and sediment samples, using selective extraction methods to distinguish different phases, is of particular interest in exploration geochemistry to locate deeply buried mineral deposits. There are various mechanisms of binding labile elements in the secondary environment, including physical and chemical sorption, precipitation, chelation and complexation. Phases present in soils and sediments which are likely to scavenge `free' elements include amorphous Mn and Fe oxides, the humic and fulvic components of humus, and clays. This paper reviews these forms of trace elements and the methods in current use to quantify them. Examples of precision data, both for control and survey samples, are given with respect to trace elements dissolved from the `soluble organic' component of humus, Mn oxides and amorphous Fe oxides. The high sensitivity of inductively coupled plasma mass spectrometry (ICP–MS) is required to measure accurately and precisely a large suite of trace elements, especially where only small fractions of elements are dissolved by such leaches as the commercially available Enzyme and MMI (Mobile Metal Ion) extractions. The relative standard deviations (RSD) obtained for 33 elements (e.g. Ag, Cd, In, I) in the standard reference sample (SRM), TILL-2, are in the range 0.5–8% for the hydroxylamine hydrochloride (NH 2OH·HCl) leach designed to extract hydrous Fe and Mn oxides. The corresponding RSDs for elements in the reactive Mn oxide phase extracted by the Enzyme leach are in the range 3–19% except for some trace elements at levels close to detection limit (e.g. Cd, Bi). The RSDs obtained for field duplicates are inferior to those for analytical replicates (i.e. sample splits), probably a reflection of different concentrations of the host phase. In one soil survey, the Fe extracted by a 0.25 M NH 2OH·HCl leach ranged conservatively from 0.2 to 1.7% whereas the Mn extracted by the Enzyme leach varied extensively, from 0.3 to >999 ppm. In contrast, precision, at 1–7% RSD, for field duplicates was found to be comparable with that for both analytical duplicates and the SRM, LKSD-4, for elements associated with the humic and fulvic component of humus samples sieved to <177 μm.

Nature of humus in Andosols under differing vegetation in the Sierra Nevada, Mexico

A sequence of soils on the flank of the volcano Iztaccíhuatl was selected to study the effect of vegetation on the nature of the organic matter found in soils derived from recent (400 years B.P.) volcanic ash. The sequence, in order of increasing elevation, extended across three climax vegetation zones: Abies religiosa, Pinus hartwegii and the Zacatonal, an alpine grassland. No significant differences between the zones were observed in the total organic C, the humic acid:fulvic ratio or the extinction coefficient of humic acid solutions at 400 nm (E 400). In contrast, Ah horizons of the Abies zone had significantly higher E 400/E 600 ratios and lower levels of a fulvic component rich in polysaccharides than found in the other zones. Differences in the relative proportions of polysaccharide and polyphenolic components of the fulvic acid were also associated with differences in vegetation. In the Pinus zone, some organic matter characteristics of the Ah 1 horizon were significantly different from those of the Ah2 horizon, whereas no such differences were observed in the Abies or Zacatonal zone. Amounts of Al and Si extractable with oxalate suggested a low overall degree of weathering but one which increased from the Abies to the Zacatonal zone (i.e., with increasing elevation).

Determination of extractable and non‐extractable radioactivity from prairie soils treated with carboxyl‐ and ring‐labelled [14C]2,4‐D

Summary:Ring‐ and carboxyl‐labelled [14C]2,4‐D were incubated under laboratory conditions, at the 2 g/g level, in a heavy clay, sandy loam, and clay loam at 85% of field capacity and 20 1C. The soils were extracted at regular intervals for 35 days with aqaeous acidic acetonitrile, and analysed for [14C]2,4‐D and possible radioactive degradation products. Following solvent extraction, a portion of the soil residues were combusted in oxygen to determine unextracted radioactivity as [14C]carbon dioxide. The remaining soil residues were then treated with aqueous sodium hydroxide, and the radioactivity associated with the fulvic and humic soil components determined.In all soils there was a rapid decrease in the amounts of extractable radioacitivity, with only 5% of that applied being recoverable after 35 days. All recoverable radioactivity was attributable to [14C]2,4‐D, and no [14C]‐containing degradation products were observed. This loss of extractable radioactivity was accompanied by an increase in non‐extractable radioactivity. Approximately 15% of the applied radioactivity, derived from carboxyl‐labelled [14C]2,4‐D, and 30% from the ring‐labelled [14C]2,4‐D was associated with the soil in a non‐extractable form, after 35 days of incubation.After 35 days, less than 5% of the radioactivity from the carboxyl‐labelled herbicide, and less than 10% of the ringlabelled material, was associated with the fulvic components derived from the three soils. Less than 5% of the applied radioactivities were identifiable with any of the humic acid components.It was considered that during the incubation [14C]2,4‐D did not become bound or conjugated to soil components, and that non‐extractable radioactivity associated with the three soil types resulted from incorporation of radioactive degradation products, such as [14C]carbon dioxide, into soil organic matter.