Teodoro Miano

Factors controlling humification and mineralization of soil organic matter in the tropics

Abstract The first part of this review focuses on the chemical composition and morphological features that characterize primary and secondary organic resources for humification. The chemical pathways of decomposition and humification of SOM in tropical soils are discussed referring mainly to the chemical structural changes identified by using both solid-state13C nuclear magnetic resonance spectroscopy (13C NMR) of bulk soil samples and liquid-state13C NMR of chemically isolated SOM fractions. The stabilization effects and mechanisms exerted on SOM by clay minerals and sesquioxides in tropical soils are also reviewed. Successively, relevant aspects of organic matter mobilization and dissolved organic matter dynamics in temperate versus tropical ecosystems are examined. In the second part of the review, general and specific aspects of mineralization processes in relation to the chemistry of main SOM pools (labile versus stable SOM components) in the tropics are discussed. Amounts, distribution, and forms of nutrients in SOM, nutrient release from organic versus inorganic sources, nutrient cycling in natural and cultivated soils, and the contribution of SOM to cationic nutrition in tropical soils are reviewed. The final part of the review focuses on the main chemical factors that control CO2 evolution and denitrification processes during SOM mineralization in tropical areas.

CHARACTERIZATION, DIFFERENTIATION, AND CLASSIFICATION OF HUMIC SUBSTANCES BY FLUORESCENCE SPECTROSCOPY

Fifty samples of humic acids and fulvic acids isolated from various soils and soil-related materials (including paleosols, peat, leonardite, composted and earth-worm-composted organic materials, sewage sludges, and materials synthesized by soil fungi) have been investigated by fluorescence spectroscopy in the emission, excitation, and synchronous-scan excitation modes. Emission spectra are generally characterized by a unique broad band showing a maximum wavelength and relative fluorescence intensity that depend mainly on the nature and origin of the humic material. Excitation and synchronous-scan excitation spectra generally exhibit a number of peaks and shoulders of relative intensity at wavelengths that also vary according to the type and source of the humic material. The observed, distinct fluorescence properties of humic substances provide useful diagnostic criteria for distinguishing between humic and fulvic acids from the same source and between humic or fulvic acids from various sources. As a consequence, a classification of humic substances is proposed on the basis of their fluorescence behavior. Hypotheses on the chemical nature of relevant fluorescing structures in the various humic materials are suggested by comparing experimental data with corresponding fluorescence data available for defined simple molecules. The long wavelengths and low intensities measured for the major fluorescence peaks of paleosol, soil, peat, and leonardite humic acid are mainly ascribed to the presence of linearly-condensed aromatic ring and other unsaturated bond systems, capable of a high degree of conjugation and bearing electron-withdrawing substituents such as carbonyl and carboxyl groups, and to their high molecular weight units. The short wavelengths and high intensities generally measured for main fluorescence peaks of compost and earth-worm-composted humic acid and soil and peak fulvic acid are associated with the presence of simple structural components of low molecular weight, low degree of aromatic polycondensation, low levels of conjugated chromophores, and bearing of electron-donating substituents such as hydroxyls, methoxyls, and amino-groups. In the Conclusions, emphasis is given to the utility of further investigations on fluorescence behavior of humic substances.

Spectroscopic and compositional comparative characterization of I.H.S.S. reference and standard fulvic and humic acids of various origin

Abstract Eleven standard and reference fulvic acids (FA) and humic acids (HA) of aquatic and terrestrial origin from the collection of the International Humic Substances Society have been studied by chemical and spectroscopic methods. Aquatic HA appear richer in O and lower in C than terrestrial HA, while the opposite is generally true for the corresponding FA. The N content of aquatic samples is always much lower, and C/N ratio and E 4 /E 6 ratio much higher than those of terrestrial samples. The infrared spectra of all samples but soil HA are qualitatively similar one to another, but they differ mainly in the relative intensity of carboxyl, aromatic and aliphatic group bands. Fluorescence of terrestrial HA occurs at higher wavelengths than terrestrial FA and aquatic HA and FA in both emission and excitation modes, while synchronous-scan spectra appear more resolved and informative. ESR spectra are featured by resonances consistent with conjugated semiquinone free radicals and Fe(III) ions, the latter resonance being generally more intense for terrestrial than for aquatic samples.

Synchronous excitation fluorescence spectroscopy applied to soil humic substances chemistry

Nine fulvic (FA) and humic (HA) acids were isolated from different soils and investigated by means of synchronous-scan excitation fluorescence spectroscopy at various Δλ = λem − λexc, in aqueous solution over a wide range of concentration and pH values. FA were characterized by more structured and resolved spectra than HA, but both showed typical features having wavelength and relative intensity variable with Δλ, concentration and pH. The short wavelength range shown by the spectra of FA was consistent with the lower degree of aromaticity and polycondensation of phenolic aromatic units with respect to the HA. A direct relationship was shown in FA spectra between the solution concentration and the relative fluorescence intensity within the entire range, whereas a maximum fluorescence intensity was measured in the spectra of HA for concentration levels ranging between 50 and 70 mg/l. In general, an increase in pH resulted in a decrease of the overall fluorescence intensity of FA spectra, and of the peak at about 475 nm in the spectra of HA samples.

Chapter 14 – Humic-like Substances in Organic Amendments and Effects on Native Soil Humic Substances

Publisher Summary This chapter reviews the chemical and physicochemical data available for the evaluation on a molecular scale of the humic substances (HS) like components in organic amendments of various origins and nature, and the effects of added HS-like materials on the composition, structure, and chemical reactivity of native soil HS. The organic amendments are discussed based on four classes: sewage sludge; municipal solid wastes; animal manures; and plant, wood, and food industry residues. Soil application of mature, stabilized, and partially humified organic wastes of any nature and origin affects the composition, structure, and chemistry of native soil HS. These modifications are more extensive and lasting in the fulvic acid (FA) fraction than in the humic acid (HA) fraction of an amended soil. The proteinaceous, S-containing, lignin, and aliphatic structures contained in HS of organic amendments are extensively incorporated into native soil HS in forms relatively resistant to further decomposition. The chapter also summarizes the scope of the treatment processes of organic wastes intended to produce soil amendments with beneficial properties for agriculture.

Heavy metal concentrations in soils as determined by laser-induced breakdown spectroscopy (LIBS), with special emphasis on chromium

Soil is unanimously considered as one of the most important sink of heavy metals released by human activities. Heavy metal analysis of natural and polluted soils is generally conducted by the use of atomic absorption spectroscopy (AAS) or inductively coupled plasma optical emission spectroscopy (ICP-OES) on adequately obtained soil extracts. Although in recent years the emergent technique of laser-induced breakdown spectroscopy (LIBS) has been applied widely and with increasing success for the qualitative and quantitative analyses of a number of heavy metals in soil matrices with relevant simplification of the conventional methodologies, the technique still requires further confirmation before it can be applied fully successfully in soil analyses. The main objective of this work was to demonstrate that new developments in LIBS technique are able to provide reliable qualitative and quantitative analytical evaluation of several heavy metals in soils, with special focus on the element chromium (Cr), and with reference to the concentrations measured by conventional ICP spectroscopy. The preliminary qualitative LIBS analysis of five soil samples and one sewage sludge sample has allowed the detection of a number of elements including Al, Ca, Cr, Cu, Fe, Mg, Mn, Pb, Si, Ti, V and Zn. Of these, a quantitative analysis was also possible for the elements Cr, Cu, Pb, V and Zn based on the obtained linearity of the calibration curves constructed for each heavy metal, i.e., the proportionality between the intensity of the LIBS emission peaks and the concentration of each heavy metal in the sample measured by ICP. In particular, a triplet of emission lines for Cr could be used for its quantitative measurement. The consistency of experiments made on various samples was supported by the same characteristics of the laser-induced plasma (LIP), i.e., the typical linear distribution confirming the existence of local thermodynamic equilibrium (LTE) condition, and similar excitation temperatures and comparable electron number density measured for all samples. An index of the anthropogenic contribution of Cr in polluted soils was calculated in comparison to a non-polluted reference soil. Thus, the intensity ratios of the emission lines of heavy metal can be used to detect in few minutes the polluted areas for which a more detailed sampling and analysis can be useful.

Characterization of solid and aqueous phases of a peat bog profile using molecular fluorescence spectroscopy, ESR and FT-IR, and comparison with physical properties

Abstract A peat core (10×10×100 cm) was removed from Etang de la Gruere, an ombrotrophic peat bog in the Jura Mountains of Switzerland. The core was sliced into 3 cm sections and the samples characterized with respect to ash content, bulk density, moisture content, and concentrations of C and H. Solid peat residues were freeze-dried, milled very finely and analyzed using Fourier transform infrared (FT-IR) and electron spin resonance (ESR) spectroscopy. Pore waters expressed from the peat slices were collected, filtered (0.2 μm membrane filters) and analyzed using fluorescence emission, excitation and synchronous-scan spectroscopy, in addition to pH. Taken together, the physical properties of the core and the ESR measurements of solid peats appear to record the variation in humification of refractory organic matter (peat). The fluorescence and FT-IR data, on the other hand, appear to characterize the extent of recent decomposition of labile organic matter. In fact, these data indicate the presence of two distinct zones in the bog: (i) the acrotelm, or zone of active decay, which may be oxygenated, depending on the season and the depth to the water table, and (ii) the catotelm which is always anoxic, and where organic matter decomposition rates are negligible.

Nature and origin of organic matter in carbonates from speleothems, marine cements and coral skeletons

Organic matter in speleothem calcite, marine carbonate cements and aragonitic coral skeletons was studied to determine its location, molecular structure, functionality and effect on mineral growth. SEM analyses showed that inorganically precipitated carbonates incorporate, during growth, adsorbed organic matter between submicroscopic subunits of the crystals whereas biologically secreted carbonates incorporate biogenic tissue between the crystals. Molecular fluorescence spectroscopy indicated that low molecular weight fulvic acids are the most important constituents of the organic matter. The fulvic acids are probably derived from soils (speleothem calcite), dissolved organic matter (marine carbonates) and biological decay products (aragonitic coral skeleton).

Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe3+ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.

Zinc distribution and speciation within rocket plants (Eruca vesicaria L. Cavalieri) grown on a polluted soil amended with compost as determined by XRF microtomography and Micro-Xanes

Zinc distribution and speciation within different organs (root, petiole, and leaf) of the edible plant Eruca vesicaria L. Cavalieri were determined using synchrotron microbeam X-ray techniques (XRF microtomography and mu-XANES) for plants grown in polluted soil with or without compost amendment. Data on soil derived from different extraction procedures and using mu-XANES analyses on rhizospheric soil indicated that compost amendment did not significantly influence the Zn speciation and availability in soil. However, major differences were observed within the plants. Plants grown in the presence of compost were able to partly block zinc immediately outside the root endodermis in the form of zinc-phytate, while a smaller Zn fraction was allowed to xylem transport as zinc-citrate. In the leaves, zinc was largely excluded from leaf cells, and about approximately 50% was in the form of phosphate precipitates, and the other 50% was complexed by cysteine and histidine residues. The reported data provide new information concerning the mechanisms of zinc tolerance in E. vesicaria L. Cavalieri, a very common edible plant in Mediterranean regions, and on the role of compost in influencing the molecular strategies involved in zinc uptake and detoxification.