Soil Humic Substances Research Articles

Spectroscopic characteristics of humates isolated from different soils

  Spectral characterisation of soil humic substances is one of the important methods for their quality identification. In this work, two optical methods (UV-VIS and SFS) were used. The absorbance in the spectral range of 300–700 nm was measured using spectrometer Varian Cary 50 Probe. Fluorescence (SFS) in the range of 255–655 nm was performed by spectrofluorimeter Aminco Bowman. Five Czech soil humates samples (Leptic Cambisol, Haplic Cambisol, Eutric Cambisol 1-arable soil, Eutric Cambisol 2-grassland, Haplic Chernozem) were compared. The basic soil properties were determined by the commonly used methods. Colour indexes (Q<sub>4/6</sub>) were calculated from the absorbance of humic substances in UV- VIS spectral range. Fluorescence indexes (F) were calculated from SFS spectra at Δλ = 55 nm (as a ratio RFI<sub>468</sub>/RFI<sub>522</sub>). Also, the classical method of humic substances fractionation to assess their quality was applied. The comparison is given of the calculated parameters from different spectral regions and humic substances fractionation. The results showed linear correlation between carbon content (C weigth %) and fluorescence indexes (R<sup>2 </sup>= 0.91), between total organic carbon content in soil and fluorescence indexes (R<sup>2 </sup>= 0.92), as well as between colour indexes (Q<sub>4/6</sub>) and humic substances content. The same main fluorophores at the wavelengths λ<sub>ex</sub>/λ<sub>em</sub>= 467/522 nm were observed in all soil humates samples.

Chemical structure of humic acids in biosolids-amended soils as revealed by NMR spectroscopy

We used NMR spectroscopy to characterize humid acids extracted from soils that had received long-term application of 2 levels of biosolids to evaluate the soil organic matter (SOM) stability in biosolids-amended soils. The study also quantified fulvic acids (FAs), humic acids (HAs) and Fe/Al oxides. The soils were collected in 2004 from 7 fields, in Fulton County, southwestern Illinois, which received biosolids at a cumulative rate of 0 (control), 554 (low biosolids) and 1,066 (high biosolids) Mg ha −1. The application of biosolids increased both FA and HA contents, but biosolids-amended soil and control soil did not differ in FA/HA ratio. Biosolids application had no effect on water-soluble organic carbon content. Biosolids application increased the presence of Fe/Al in the SOM complex and lowered its C/Fe and C/Al ratios. 13C NMR spectra showed increased alkyl C and decreased aromatic C content in soil HAs with the application of biosolids, and the extent of such changes was higher with high than low biosolids treatment. Under biosolids application, the soil HAs’ C structure shifts from O-alkyl-dominant to alkyl-dominant. Biosolids application does not decrease SOM stability but rather increases the stability of soil humic substances.

Modeling Apparent Nitrogen Mineralization under Field Conditions Using Regressions and Artificial Neural Networks

Soil N mineralization is an important source of N for grain crops, but its estimation under field conditions is usually very difficult. Our objective was to develop models suitable for predicting N mineralization during the growing seasons of wheat (Triticum aestivum L.) and corn (Zea mays L.) under field conditions. Fifty‐eight field experiments were performed with wheat, and 35 with corn, along three growing seasons, in which soil apparent N mineralization was estimated by the mass balance approach. Apparent nitrogen mineralized from decomposing residues (ANMR) or soil humic substances (ANMH) were estimated separately. Two empirical modeling techniques were tested, linear regression and artificial neural networks, using as independent variables or inputs some environmental variables. Both techniques allowed the development of suitable models for N mineralization prediction (R2 > 0.68), but neural networks gave slightly better results. The ANMR ranged from −42 to 64 kg N ha−1, increasing as residue mass and N concentration increased. An average ANMR of 15 to 16 kg N ha−1 was produced both during wheat and corn growing seasons. The ANMH ranged from −80 to 328 kg N ha−1, being on average four times greater during corn growing cycle than during wheat season (127 vs. 34 kg N ha−1). The ANMH decreased as initial mineral N content of the soil, remaining residue mass or fine particles content of the soil increased, and it was greater in soils of higher organic matter level and mineralization potential, as determined by an incubation test. Increases in temperature and rainfall also determine greater ANMH. The methodology developed for apparent N mineralization estimation may be applied to other crops and production regions.

Effects of nanostructural organization of soil gels on the fractional composition of soil humic substances

Effects of nanostructural organization of soil gels on the fractional composition of soil humic substances

Humic acid model substances with pronounced redox functionality for the study of environmentally relevant interaction processes of metal ions in the presence of humic acid

Humic acid (HA) model substances with pronounced redox functionality were synthesized by oxidation of hydroquinone or catechol in the presence of glycine or glutamic acid and characterized concerning their elemental, structural and functional properties. In order to characterize the redox properties of the synthetic products, formal redox potentials and Fe(III) reducing capacities were determined and compared to purified Aldrich HA (AHA). Furthermore, the reduction of U(VI) in the presence of HA was studied. The synthetic products show elemental, functional and structural properties comparable to natural HA, however, they are characterized by high amounts of phenolic/acidic OH groups (5.3–6.6 meq/g). Furthermore, the synthetic HA show significantly higher reducing capacities for Fe 3+ and [Fe(CN) 6] 3− at pH 3.0 (8.8–14.5 meq/g) and at pH 9.2 (27.5–36.9 meq/g), respectively, than AHA (pH 3.0: 1.2 ± 0.1 meq/g; pH 9.2: 7.2 ± 1.9 meq/g). The highest reducing capacities were obtained for HA Cat-Gly (pH 3.0: 14.5 ± 1.6 meq/g; pH 9.2: 36.9 ± 0.2 meq/g), an oxidation product from catechol and glycine, which is characterized by the lowest formal redox potential (E 0⁎ = 517 ± 12 mV) of all studied HA (E 0⁎ = 517–571 mV). Indications for a slight reduction of U(VI) in the presence of HA were observed, whereby, HA Cat-Gly exhibits again the highest reducing capacity (pH 6: 0.065 ± 0.002 meq/g). Using modified HA with blocked phenolic/acidic OH groups the importance of these functional groups for the redox behavior of HA was confirmed. Synthetic HA with pronounced redox functionality can be used to study the redox behavior of HA and the redox stability of metal ions in the presence of HA and furthermore, to stabilize redox-sensitive metal ions against oxidation in complexation and transport studies with HA. This contributes to a better understanding of interaction processes of metal ions with humic substances in soils, sediments and waters.

Effect of Fusarium oxysporum f. sp. lycopersici on the Soil-to-Root Translocation of Heavy Metals in Tomato Plants Susceptible and Resistant to the Fungus

The purpose of this work was to gain an insight on the potential role of the phytopathogenic fungus Fusarium oxysporum f. sp. lycopersici in the translocation of metals and metalloids from soil to plant roots in tomato (Lycopersicum esculentum). Two varieties of tomato (one susceptible and another resistant to infection by Fusarium oxysporum f. sp. lycopersici) were challenged with the fungus for different periods of time, and several elements (V, Cr, Mn, Co, Cu, Zn, As, Se, Mo, Ag, Cd, Pb) were determined in roots and in soil substrate. Additionally, phenolic plant products were also analyzed for the evaluation of the plant response to biotic stress. In order to obtain representative results for plants cultivated in noncontaminated environments, the infected and control plants were grown in commercial soil with natural, relatively low metal concentrations, partly associated with humic substances. Using such an experimental design, a specific role of the fungus could be observed, while possible effects of plant exposure to elevated concentrations of heavy metals were avoided. In the infected plants of two varieties, the root concentrations of several metals/metalloids were increased compared to control plants; however, the results obtained for elements and for phenolic compounds were significantly different in the two plant varieties. It is proposed that both Lycopersicum esculentum colonization by Fusarium oxysporum f. sp. lycopersici and the increase of metal bioavailability due to fungus-assisted solubilization of soil humic substances contribute to element traffic from soil to roots in tomato plant.

The influence of reduced tillage, winter crops and ecologically managed long-term mono- and multi-component swards on soil humic substances

Chemical analyses of soil humic substances were performed at the Chemical Research Laboratory of the LIA. A field experiment (Experiment 1) was established on a glacio-lacustrine clay loam on a silty clay Endocalcari–Endohypogleyic Cambisol. The study investigated two soil tillage systems: conventional (CT) and reduced (RT). The study also encompassed crop rotations with different structures: 0, 25, 50, 75 and 100% winter crops in a crop rotation. Experiment 2 compared the influence of long-term legume swards on soil humic substances in an Epicalcari–Endohypogleic Cambisol. The swards were used for five years under ecological management. No fertilisers or pesticides were used. There were 2.6 times more labile humic acids (HA) in the topsoil in RT compared with CT. A significant effect on the increase of labile HA content was determined in crop rotations with≥50% winter crops. The highest amount of mobile HA accumulated in soil under a four-component sward (Galega+Trifolium repens+Onobrychis+Festulolium). Reduced soil use resulted in qualitative changes in HA determined by Fourier transform infrared (FT-IR) spectroscopy. The analytical methods used are appropriate for evaluating sensitivity to changes in soil management and are helpful for the development of reduced soil use systems. However, labile and mobile carbon pools are more sensitive than humus.

Altered microbial community structure and organic matter composition under elevated CO2 and N fertilization in the duke forest

AbstractThe dynamics and fate of terrestrial organic matter (OM) under elevated atmospheric CO2 and nitrogen (N) fertilization are important aspects of long‐term carbon sequestration. Despite numerous studies, questions still remain as to whether the chemical composition of OM may alter with these environmental changes. In this study, we employed molecular‐level methods to investigate the composition and degradation of various OM components in the forest floor (O horizon) and mineral soil (0–15 cm) from the Duke forest free air CO2 enrichment (FACE) experiment. We measured microbial responses to elevated CO2 and N fertilization in the mineral soil using phospholipid fatty acid (PLFA) profiles. Increased fresh carbon inputs into the forest floor under elevated CO2 were observed at the molecular‐level by two degradation parameters of plant‐derived steroids and cutin‐derived compounds. The ratios of fungal to bacterial PLFAs and Gram‐negative to Gram‐positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin‐derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. 1H nuclear magnetic resonance (NMR) spectra of soil humic substances revealed an enrichment of leaf‐derived alkyl structures with both elevated CO2 and N fertilization. We suggest that microbial decomposition of SOM constituents such as lignin and hydrolysable lipids was promoted under both elevated CO2 and N fertilization, which led to the enrichment of plant‐derived recalcitrant structures (such as alkyl carbon) in the soil.

Humus composition and the structural characteristics of humic substances in soils under different land uses in Leyte, Philippines

The aim of the present study was to describe the effects of land use on humus composition and the structural properties of humic substances (HS) in degraded soil in Leyte, Philippines. Five adjacent land-use types, primary forest (PF), mahogany plantation (MP), rainforestation farming (RF), coffee plantation (CP) and grassland (GR), with comparable geology, parent material, soil type and climate were sampled and examined. The contents of humic acids (HAs), fulvic acids (FAs), polyvinylpyrrolidone (PVP)-adsorbed FAs and PVP-non-adsorbed FAs in the soil, and the elementary composition, infrared (IR) spectra and molecular size distribution of HAs and PVP-adsorbed FAs were compared among the different land uses. Results revealed that conversion of PF into other land uses has led to a decrease in FAs, which were higher under intensive cultivation conditions, such as RF, CP and GR. The amounts of HAs were small and also affected by land use. The amount of PVP-non-adsorbed FAs was more affected by land use than the amount of PVP-adsorbed FAs. Variation in the molecular size of HAs and PVP-adsorbed FAs provided evidence of variations in the structural characteristics of HS as a result of land-use change, although there were no appreciable differences in IR spectra and elementary composition. The relationship between land use and the degree of humification of HAs was poor. The HAs were still in the early stage of humification as indicated by the Type Rp in most soils, the higher contents of elemental H and N, and the high H/C and O/C ratios.

Fractionation of copper and cadmium and their binding with soil organic matter in a contaminated soil amended with organic materials

Purpose The contamination of agricultural soils by heavy metals is a worldwide problem. Organic amendments can be used for the immobilization and binding of heavy metal ions in soils by complexation, adsorption, and precipitation. A field trial was carried out to evaluate the influence of some low-cost organic materials such as rice straw (RS), green manure (GM), and pig manure (PM) on the distribution of Cu and Cd and the retention of these metals by organic matter fractions in heavy metal-polluted soils. Materials and methods The experiment was conducted in Miaoyunao Village, Daye County, Hubei province, China. PM, GM (peanut plants), and RS were obtained from a farm close to the village. Sixteen treatments with three replicates were designed. Soil chemical properties such as soil pH, electrical conductivity (EC), organic matter (OM), and available P were measured by standard methods. Soluble/ exchangeable, organic-bound, inorganic precipitates and residual Cu and Cd in the soil were sequentially extracted and analyzed. The amounts of Cu and Cd bound with soil particulate organic matter (POM) fractions and humic substances were also determined. Results and discussion The addition of organic amendments declined significantly the concentrations of soluble/ exchangeable Cu and Cd, but increased the amounts of these metals in organic-bound and inorganic precipitate forms in the soil. RS was more effective than GM and PM in diminishing the solubility of Cu and Cd. The largest retention for Cu and Cd by humic substances and POM was noticedinRStreatments,whereasthelowestwasfoundinPM treatments. Humic substances showed higher potential in the fixation of Cu and Cd than POM fractions. The conversion of soluble/exchangeable Cu and Cd to other insoluble forms after the application of organic amendments may be ascribed to the increases of soil OM, pH, EC, and available P contents. The highest binding of Cu and Cd with POM fractions and humic substances after the incorporation of RS mainly resulted from the greatest increase of soil OM contents. Conclusions RS, GM, and PM can be employed as good and cheap substances for the immobilization of Cu and Cd in heavy metal-polluted soils. RS was the best amendment in decreasing the solubility of Cu and Cd, and also in enhancing the retention of these metals by humic substances and POM fractions in the soil. Futures studies should focus on the influence of these organic amendments or their mixtures on the phytotoxicity of Cu and Cd for different plants in heavy metal-contaminated soils.

Influence of the humification process on the properties of peat humic acids

Influence of the humification process on the properties of peat humic acids Comparative and complex characterisation of peat humic acids (HAs) isolated from peat profiles of different origin in Latvia was conducted. Elemental and functional analysis of the isolated HAs was made and their acidity and molecular weight were estimated. Spectral characterisation included UV-Vis, IR, electron spin resonance and fluorescence spectra. Structural characterisation of HAs was by both 1H and13C nuclear magnetic resonance spectra. Properties of HAs isolated from the Latvian peat were compared with HA from other sources (soil, water, coal and synthetic humic substances). Major properties of peat HAs depended on their origin, indicating the importance of humification processes. HAs isolated from peat of more recent origin were more similar to soil Has, while there was a greater degree of humification in part from older sources.

Characterization by Fluorescence of Organic Matter from Oxisols under Sewage Sludge Applications

Sewage sludge from wastewater treatment contains organic matter and plant nutrients that can play an important role in agricultural production and the maintenance of soil fertility. The present study has aimed to evaluate the degree of humification following sewage sludge application of soil organic matter by laser‐induced fluorescence and humic acids using ultraviolet‐visible fluorescence, and including comparison with Fourier‐transform infrared spectroscopy and elemental analysis. Sewage sludge applications to the soil caused a decrease in the degree of humification of the soil organic matter and humic acids for both a Typic Eutrorthox (clayey) soil and a Typic Haplorthox (sandy) soil of around 14 and 27%, respectively. This effect is probably due to incorporation of newly formed humic substances from the sewage sludge into the characteristics of less humified material, and to the indigenous soil humic substances. The minor alterations observed in the clay soil probably occurred due to both the greater mineral association, which better stabilized the indigenous soil organic matter, and the higher microbial activity in this soil, which accelerated sewage sludge mineralization. Sewage sludge applications increased the C content for the clay and sandy soils by 7.4 and 15.4 g kg−1, respectively, suggesting a positive effect on these two soils.

Selected pioneering works on humus in soils and sediments during the 20th century: A retrospective look from the International Humic Substances Society view

Organic matter in general, and humic substances (HS) in particular, are involved in many processes in soils, sediments, rocks and natural waters. These include rock weathering, plant nutrition, pH buffering, trace metal mobility and toxicity, bioavailability, degradation and transport of hydrophobic organic chemicals, formation of disinfection by-products during water treatment, heterotrophic production in blackwater ecosystems and, more generally, the global carbon cycle. Before the 1970s, natural organic matter of different ecosystem pools ( i.e., soils, sediments, and natural waters) was often studied in isolation, although many similarities exist between them. This is particularly so for HS. In this historical context, a need appeared at the international level for bringing together environmental chemists, soil scientists, hydrologists, and geologists who were interested in HS to provide a forum for the exchange of ideas, to standardize analytical procedures and agree on definitions of HS. The International Humic Substances Society (IHSS) was founded in Denver, Colorado (USA) in 1981 with several objectives among them “to bring together scientists in the coal, soil, and water sciences with interests in humic substances” (home page of the IHSS web site: http://ihss.gatech.edu/ihss2/index.html). This paper presents selected pioneering works on humus in soils and sediments during the 20th century with a special focus on the links between the studies of soil HS and the formation, during early diagenesis, of the precursors of kerogens. Temporal coverage includes key contributions preceding the founding of the IHSS, and a brief history of the organization is presented.

A comparison of extraction techniques on the stable carbon-isotope composition of soil humic substances

Because humic substances have varying turnover rates, their stable carbon-isotope compositions have been used to assess carbon residence time in soils. However, any study of this nature must ensure that extraction techniques do not fractionate the δ 13C values of the humic acid extract and residual humin. This study examines the effects of different extraction procedures on the carbon-isotope signature of the humic fractions of soil organic matter. Humic acid and humin were isolated from soils using three different treatment methods: extraction with 0.5 M NaOH solution until completion, extraction with 0.1 M NaOH + Na 4P 2O 7 solution until completion, and extraction with 0.1 M NaOH + Na 4P 2O 7 solution for only one day. The δ 13C values of the humin fractions were similar and the δ 13C values of the humic acid fractions were similar regardless of extraction technique when the soil carbon content was greater than 0.5% prior to extraction and in many soils when carbon content was less than 0.5%. However, within the group of soils that had a low carbon content some soils exhibited shifts in the δ 13C values of the humic acid or humin when different extraction techniques were used. In these samples, humic acids extracted after only one day of reaction were slightly enriched in 13C relative to those that were treated until exhaustion with 0.1 M NaOH + Na 4P 2O 7. Where shifts in δ 13C values for humic acid or humin fractions were observed in low carbon fractions treated with different chemical solutions, those fractions extracted with 0.5 M NaOH were enriched in 13C by 0.5 to 2.3‰ relative to those extracted with the sodium pyrophosphate mixture, with one exception. Possible explanations for 13C enrichment of humic acids produced with a NaOH extraction solution are discussed including contamination of the soluble humic acid with non-humified carbon or carbon associated with solubilised silica. Samples at the greatest depth in each of the soil profiles displayed widely variable behaviour for the δ 13C values of humic acid or humin purified with the different chemical solutions. This study demonstrates that for some of the samples with low carbon contents δ 13C values of humin or humic acid can be either enriched or depleted in 13C, when different extraction solutions are used.

Effect of long‐term combined nitrogen and phosphorus fertilizer application on 13C CPMAS NMR spectra of humin in a Typic Hapludoll of northeast China

SummaryBecause of its insolubility, heterogeneity and structural complexity, humin is the least understood among the three fractions of soil humic substances. This research aimed to evaluate the long‐term effect of combined nitrogen and phosphorus (NP) fertilizer addition on the chemical structure of humin under maize (Zea mays L.) monoculture in a Typic Hapludoll of northeast China. Soil samples were collected 12 and 25 years after the initiation of the fertilizer treatment. Soil humin was isolated using NaOH‐Na4P2O7 extraction to remove humic and fulvic acids, which was followed by HF‐HCl treatment to remove most of the inorganic minerals. Solid‐state 13C cross‐polarization magic angle spinning nuclear magnetic resonance (13C CPMAS NMR) spectroscopy was used to characterize the chemical structure of the humin isolates. Results showed that the organic carbon (C) content of humin increased after NP fertilizer addition, compared with a no‐fertilizer (CK) treatment. 13C CPMAS NMR indicated that O‐alkyl C and aromatic C of humin decreased, while alkyl C and the ratios of alkyl C/O‐alkyl C, aliphatic C/aromatic C and hydrophobic C/hydrophilic C all increased in the NP fertilizer treatment. The long‐term application of NP fertilizer changed the molecular structure of soil humin to be more alkyl and hydrophobic, and was thus beneficial to the sequestration and stability of organic C in soil.

Characterizing humic substances from estuarine soils and sediments by excitation-emission matrix spectroscopy and parallel factor analysis

The determination of optical properties of organic matter using spectroscopic techniques is a powerful tool for the characterization of humic substances (HS) in soils and sediments because of sensitivity, specificity and sample throughput. However, basic spectroscopic techniques have practical limitations because of the similarity in the optical properties of many HS. To improve resolution, the combination of excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC) was applied for characterizing fulvic acid (FA) and humic acid (HA) fractions from soils and sediments of two estuarine environments in Spain. Five fluorescent components were identified by EEM-PARAFAC and were found in both FA and HA fractions, consistent with the new paradigm of HS as supramolecular associations as well as the ubiquity of the HS components in the environment. Their contribution was, however, different between the FA and HA fractions. Two different, humic-like, fluorescent components were representative of FA and HA fractions, respectively. The spectral characteristics of these components were similar to previously reported PARAFAC components in dissolved organic matter (DOM) in a wide range of environments, suggesting their applicability in assessing OM quality and environmental dynamics. A microbial humic-like component was much more abundant in FA than in HA fractions. Furthermore, principal component analysis clarified that the two identified protein-like components, were enriched in sediment HA compared to soil HA, suggesting a larger contribution of refractory algaenan in sediment HA. The results of the present study demonstrate that EEM-PARAFAC is a useful technique for the biogeochemical characterization of soil and sedimentary HS.

Biosurfactants from Urban Green Waste

From waste came forth surfactants: Humic acid like substances isolated from 0-60 day-old compost display excellent surface activity and solvent properties. These biosurfactants were used to solubilize a dye in water below and above their critical micellar concentration. The biosurfactant unimers appear to have higher dye-solubilizing power than the corresponding micelles.Humic acid like substances isolated from compost show potential as chemical auxiliaries. In the present study, three surfactant samples were obtained from green waste composted for 0-60 days to assess aging effects of the source on the properties of the products. The surface activity, dye solubility enhancement, and chemical nature of these substances were compared. No differences in performance were established among the samples. They lower water surface tension and enhance the dye solubility upon increasing their concentration. However, the ratio of soluble dye to added surfactant is higher in the premicellar than in the postmicellar concentration region. Structural investigations indicated the humic acid like substances to be amphiphiles with molecular weights in the range of 1-3 x 10(5) g mol(-1). The surfactant samples were also compared to sodium dodecylbenzenesulfonate, polyacrylic acid, and soil and water humic substances. The results encourage the application of compost as a source of low-cost biosurfactant.

N-15 NMR spectra of naturally abundant nitrogen in soil and aquatic natural organic matter samples of the International Humic Substances Society

The naturally abundant nitrogen in soil and aquatic NOM samples from the International Humic Substances Society has been characterized by solid state CP/MAS 15N NMR. Soil samples include humic and fulvic acids from the Elliot soil, Minnesota Waskish peat and Florida Pahokee peat, as well as the Summit Hill soil humic acid and the Leonardite humic acid. Aquatic samples include Suwannee River humic, fulvic and reverse osmosis isolates, Nordic humic and fulvic acids and Pony Lake fulvic acid. Additionally, Nordic and Suwannee River XAD-4 acids and Suwannee River hydrophobic neutral fractions were analyzed. Similar to literature reports, amide/aminoquinone nitrogens comprised the major peaks in the solid state spectra of the soil humic and fulvic acids, along with heterocyclic and amino sugar/terminal amino acid nitrogens. Spectra of aquatic samples, including the XAD-4 acids, contain resolved heterocyclic nitrogen peaks in addition to the amide nitrogens. The spectrum of the nitrogen enriched, microbially derived Pony Lake, Antarctica fulvic acid, appeared to contain resonances in the region of pyrazine, imine and/or pyridine nitrogens, which have not been observed previously in soil or aquatic humic substances by 15N NMR. Liquid state 15N NMR experiments were also recorded on the Elliot soil humic acid and Pony Lake fulvic acid, both to examine the feasibility of the techniques, and to determine whether improvements in resolution over the solid state could be realized. For both samples, polarization transfer (DEPT) and indirect detection ( 1H– 15N gHSQC) spectra revealed greater resolution among nitrogens directly bonded to protons. The amide/aminoquinone nitrogens could also be observed by direct detection experiments.

Humic substances in estuarine soils colonized by Spartina maritima

This study characterizes humic substances (HS) from two soils colonized by Spartina maritima at different physiographical positions in estuarine environments on the north-western coast of the Iberian Peninsula: the Villaviciosa site, a stand close to the main tidal channel, and the Ortigueira site, located in the low salt marsh. Humic and fulvic acids were extracted from the soils and characterized qualitatively by the following spectroscopic techniques: Fourier transform infrared spectroscopy, fluorescence spectroscopy and solid-state 13C nuclear magnetic resonance. The characterized HS showed a predominance of low humified compounds with a high proportion of aliphatic components and a low degree of aromaticity. The HS composition differed substantially between sites. In the Villaviciosa soil, the large amount of nonpolar aliphatic components and the very low degree of aromaticity may indicate a significant contribution of marine organic matter and/or microbial material to the HS. However, in the HS from the Ortigueira soil, the higher proportion of polysaccharides together with the presence of lignin-derived compounds may indicate greater inputs of vascular plant material. The δ 13C isotopic composition of the bulk soils highlights the large input of Spartina maritima debris to the Ortigueira site, whereas in the Villaviciosa site, organic contributions from this C4 vascular plant were not so evident. The results indicate that in these soils colonized by Spartina maritima, physiographical position has an important effect on the composition of soil HS and, therefore, must be considered in the study of organic matter characteristics in such estuarine environments.

Surfactive stabilization of multi-walled carbon nanotube dispersions with dissolved humic substances

Soil humic substances (HS) stabilize carbon nanotube (CNT) dispersions, a mechanism we hypothesized arose from the surfactive nature of HS. Experiments dispersing multi-walled CNT in solutions of dissolved Aldrich humic acid (HA) or water-extractable Catlin soil HS demonstrated enhanced stability at 150 and 300 mg L −1 added Aldrich HA and Catlin HS, respectively, corresponding with decreased CNT mean particle diameter (MPD) and polydispersivity (PD) of 250 nm and 0.3 for Aldrich HA and 450 nm and 0.35 for Catlin HS. Analogous trends in MPD and PD were observed with addition of the surfactants Brij 35, Triton X-405, and SDS, corresponding to surfactant sorption maximum. NEXAFS characterization showed that Aldrich HA contained highly surfactive domains while Catlin soil possessed a mostly carbohydrate-based structure. This work demonstrates that the chemical structure of humic materials in natural waters is directly linked to their surfactive ability to disperse CNT released into the environment.