N. Senesi

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.

Molecular and quantitative aspects of the chemistry of fulvic acid and its interactions with metal ions and organic chemicals. Part II. The fluorescence spectroscopy approach.

Abstract Information in the literature on the use of fluorescence spectroscopy for the study of the chemistry of fulvic acid and its interactions with metal ions and organic chemicals is discussed. Basic principles, instrumentation, procedures, methodology and limitations of the fluorescence technique and the fluorescence polarization method are briefly summarized. This is followed by an extended discussion on the direct information that fluorescence spectra can furnish on the properties and nature of fluorescing structures in fulvic acids. The effects of molecular parameters and environmental factors (molecular weight, concentration, pH, ionic strength, temperature and redox potential) on the fluorescence behaviour of fulvic acids are discussed. Particular attention is devoted to the indirect information that fluorescence quenching and fluorescence polarization studies provide on molecular and quantitative aspects of the chemistry of fulvic acid in solution, especially in relation to molecular conformation and binding with metal ions and organic chemicals. The potential advantage of this non-separative, non-destructive technique for the study of environmental samples such as fulvic acids is emphasized.

Composted materials as organic fertilizers

Abstract The subject-matter of this review paper is divided into three sections. The first section briefly discusses on input, transformation and loss of native and added organic matter in soil and on principal effects of fresh organic matter addition on soil properties. The subject of the second section focuses on organic compost. The third section discusses extensively on the most commonly used waste composting process, substrate materials and products of compostation, i.e. criteria and parameters for the evaluation of compost quality as organic fertilizers, including various proposed “humification degree” indexes and compositional, structural and functional properties of humic-like substances in compost. The paper ends with some conclusive comments and recommendations.

Binding mechanisms of pesticides to soil humic substances

This review-paper summarizes and discusses the nature of the binding forces involved and the types of mechanisms operating, often simultaneously, in the adsorption processes of several pesticides onto soil humic substances, humic acids and fulvic acids. These include ionic, hydrogen and covalent bonding, charge-transfer or electron donor-acceptor mechanisms, Van der Waals forces, ligand exchange, and hydrophobic bonding or partitioning. Experimental evidence obtained and interpretation provided for the various adsorption processes proposed are briefly presented and commented. The review ends with some concluding remarks and recommendations for future work needed to be done.

TRACE ELEMENT INPUTS INTO SOILS BY ANTHROPOGENIC ACTIVITIES AND IMPLICATIONS FOR HUMAN HEALTH

Trace element definition and functions, and inputs into soils from the most important anthropogenic sources, related and not related to agricultural practices, of general and local or incidental concern, are discussed in the first part of this review. Trace element inputs include those from commercial fertilizers, liming materials and agrochemicals, sewage sludges and other wastes used as soil amendments, irrigation waters, and atmospheric depositions from urban, industrial, and other sources. In the second part of the review, the most important ascertained effects of soil trace elements on human health are presented. The possible relations found between some specific soil trace elements, such as Cd, Se, As and others, and cancer incidence and mortality, and diffusion of other important human diseases are reviewed. Brief conclusions and recommendations conclude this review.

Molecular and quantitative aspects of the chemistry of fulvic acid and its interactions with metal ions and organic chemicals : Part I. The electron spin resonance approach

Abstract A review is presented of the application of electron spin resonance (ESR) spectroscopy to the study of the chemistry of fulvic acid and its interactions with paramagnetic metal ions. After a short introduction on the nature and role of fulvic acids in the global environment, basic theory, instrumentation, procedures, methodology and limitations of ESR spectroscopy are summarized. This is followed by a discussion of the direct information that ESR data can provide on the status of organic free radical and paramagnetic metal species in fulvic acid. The effects of physical and chemical factors, including pH, ionic strength and state of aggregation, hydrolysis and alkylation, redox conditions and irradiation, on free radicals in fulvic acids are considered, also in relation to structural and reactive implications. Molecular and quantitative information furnished by ESR on both naturally occurring and laboratory-prepared paramagnetic metal ion-fulvic acid complexes are discussed. Emphasis is placed on the potential, sensitivity and advantages of this non-separative, non-destructive technique for the study of environmental samples such as fulvic acids.

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.

Humic acids in the first generation of EUROSOILS

Humic acids (HAs) were isolated from the first generation of the EUROSOILS, a reference set of five samples representing the most widespread soil units in the European Union (EU). The EUROSOIL (E)-HA were characterized for their compositional, structural, and functional properties by using chemical methods and spectroscopic techniques, including Fourier transform infrared (FT IR), nuclear magnetic resonance, fluorescence, and electron spin resonance. The E-HAs generally exhibit features typical of soil HAs of similar origin and nature, and differ in their composition, structure, and chemical properties on dependence on the parent soil type, vegetation cover, and land use. The E-HA from the Orthic Podzol under coniferous forest (E5-HA) exhibits the largest oxygen and acidic functional group contents, organic free radical concentration, aromatic polycondensation and humification degree, and the distinct presence of outer-sphere complexed Mn2+ ions. The E-HAs from the Dystric Cambisol under grassland/pasture (E3-HA) and the Orthic Luvisol cultivated for wheat (E4-HA) are generally characterized by a great content of nitrogen-containing and methoxyl groups and polysaccharide structures, small contents of acidic functional groups and free radicals, relatively small aromaticity and humification degree, and the presence of both inner-sphere complexed Cu2+ ions and outer-sphere complexed Mn2+ ions. The E-HAs from the Vertic Cambisol under grassland/meadow (E1-HA) and the Rendzina under broad-leaved tree cover (E2-HA) are characterized by properties that are generally intermediate between those of E5-HA and E3- and E4-HAs, being E1-HA more similar to E5-HA and E2-HA to E3- and E4-HAs. The differences in chemical and physicochemical properties of these E-HAs are expected to affect differently their reactivity toward soil organic and inorganic contaminants.

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.