Chemosphere 111 (2014) 184–194

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Chemical and spectroscopic characteristics of humic acids and dissolved organic matter along two Alfisol profiles Andreina Traversa a, Valeria D’Orazio b, Giuseppe Natale Mezzapesa b, Eleonora Bonifacio c, Karam Farrag d, Nicola Senesi b, Gennaro Brunetti b,⇑ a

Dipartimento di Scienze Agrarie, degli Alimenti e dell’Ambiente, University of Foggia, 71122-Foggia, Italy Dipartimento di Scienze del Suolo, della Pianta e degli Alimenti, University of Bari, 70126-Bari, Italy Dipartimento di Scienze Agrarie, Forestali e Alimentari, University of Torino, 10095-Grugliasco, TO, Italy d Biology and Environmental Indicators, Department of Central Laboratory for Environmental Quality Monitoring (CLEQM), National Water Research Centre (NWRC), Egypt b c

h i g h l i g h t s  Characterization of humic acids isolated from two Alfisol profiles.  Characterization of dissolved organic matter extracted from the profiles above.  Qualitative differences between the organic matter of the two profiles examined.  Possible interactions between the organic matter and the iron oxides in soils.

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Article history: Received 22 November 2013 Received in revised form 25 February 2014 Accepted 12 March 2014

Handling Editor: X. Cao Keywords: Alfisol Texture Soil organic matter fractions FTIR spectroscopy Fluorescence spectroscopy

a b s t r a c t The aim of this study was to elucidate the heterogeneous structural and functional composition of humic acids (HAs) and dissolved organic matter (DOM) isolated from two Alfisol profiles with different soil texture, in order to develop a better understanding of the organic matter dynamics. Soil samples were collected at different depths from three (Ap, 2AB and 2Bt) and eight (A1, A2, A3, E1, E2, 2Bt1, 2Bt2 and 2Bt3) soil horizons of two Alfisols located in the south (PR1) and north (PR2) of Italy, with a clay texture and a silt loam to loam ones, respectively. Chemical and spectroscopic methods were used to characterize the HAs and the DOM isolated from different soil horizons, including Ultraviolet–Visible (UV–Vis), Fourier Transform Infrared (FTIR), and Fluorescence spectroscopies. The HAs and the DOM isolated from the two Alfisols apparently showed significant differences in their compositional, structural and functional characteristics. In particular, the HAs isolated from the PR1 featured a higher degree of humification and molecular complexity with respect to those isolated from the PR2. On the contrary, the DOM samples isolated from the PR2 showed a more marked aromatic character and polycondensation degree. Both the HAs and the DOM obtained from the PR1 presented a greater qualitative homogeneity with respect to those obtained from the PR2. These results could be reasonably ascribed to the different texture and horizons of the two Alfisols, and to a greater pedogenesis occurred in the PR1. Ó 2014 Published by Elsevier Ltd.

1. Introduction Soil organic matter (SOM) is a complex, heterogeneous mixture of materials of various origins with a continuum of decomposition and stabilization in the soil profile (Senesi and Loffredo, 1999). Chemical stabilization of the SOM involves interactions between ⇑ Corresponding author. Address: Dipartimento di Scienze del Suolo, della Pianta e degli Alimenti, Università di Bari, Via Amendola 165/A, 70126 Bari, Italy. Tel.: +39 0805442953; fax: +39 0805442850. E-mail address: [email protected] (G. Brunetti). 0045-6535/Ó 2014 Published by Elsevier Ltd.

organic and inorganic substances that reduce the availability of the organic substrate due to metal complexation phenomena and changes in its conformation (Guggenberger and Kaiser, 2003). Iron and aluminum oxides, hydroxides, and oxyhydroxides influence the SOM stabilization (Tipping, 1981), and are thus able to retard microbial/enzymatic mineralization (Boudot et al., 1989). In particular, fine organic fractions as well as incompletely humified residues can be stabilized by sorption, entrapment, or complexation with Al- and Fe-sesquioxides, either crystalline or non-crystalline (Guggenberger and Haider, 2002). Soil texture also affects the SOM stabilization, as a result of electrostatic binding between

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negatively charged clay surfaces and organic colloids via cation bridges (Ingram and Fernandes, 2001; Paustian et al., 2003). In particular, clay-sized aggregates were found to contain older SOM (Schöning and Kögel-Knabner, 2006), hence clay mineral may contribute to preserve organic matter; as a result of that, organic matter typically decomposes more rapidly in sandy soils than in clay soils (Ladd et al., 1985). However, 2:1 layer silicates are of little importance in organic matter stabilization (Eusterhues et al., 2003) due to their large permanent charge, while phyllosilicates with prevalent variable charge, such as kaolinite, play a more important role (Bayer et al., 2001; Egli et al., 2008). Alfisols (or Luvisols) are characterized by a clay-enriched Bt horizon, typically overlaid by a clay-depleted one and lessivage is the main soil forming process (Lavkulich and Arocena, 2011). They are widespread all over the world, and occur in all soil moisture regimes (Bockheim and Hartemink, 2013) but the morphology of the profile, as well as their chemical and mineralogical characteristics, are highly variable, depending on the effects that climate has on all pedogenic processes. In particular, climate affects both textural differentiation between the E and Bt horizons and, through the interaction with soil parent material, the total amounts of fine and coarse particles. The humic acids (HAs) largely contribute to soil structure and stability, permeability for water and gases, water holding capacity, nutrient availability, pH buffering and interaction with metal ions (Schnitzer, 2000; Hayes and Malcom, 2001). The HAs differ in quantity and chemical composition according to many variables, such as climate and altitude (Martin et al., 1998), soil pH (Kloster et al., 2013), vegetation type (López et al., 2008; Traversa et al., 2011), and soil management (Bayer et al., 2002; González Pérez et al., 2004). Mahieu et al. (1999) show the influence of clay content on the rate of organic C decomposition and the proportion of decomposition products that are stabilized in soils. In particular, high clay contents ensure a physical protection of soil organic matter and hinder strong structural alterations, with direct impacts on humic acids, that, in turn, appear more complex and aromatic. The dissolved organic matter (DOM) represents an active OM reservoir in soils (Tanoue et al., 1995) and plays a key role in many chemical and biological processes that occur, which include mobilization and transport of colloids, nutrients, metals and pollutants (Jandl and Sollins, 1997). Despite these roles, the processes contributing to the production and loss of the DOM are not yet fully understood, thus making it difficult any prediction about the effects of changing environmental conditions on its properties and dynamics (Kaiser et al., 2001). The aim of this study was to assess the chemical and spectroscopic characteristics of the HAs and the DOM isolated from the various horizons of two Alfisols sampled in two contrasting climatic areas, in order to evaluate the different the SOM dynamics along their profiles.

2. Materials and methods 2.1. Soil profiles An Alfisol profile from a farm located in Rutigliano (Apulia, Southern Italy) and another one from Vauda di Nole (Piedmont, Northern Italy) were described and used for this study (Fig. 1). The Rutigliano soil profile (PR1) develops under Mediterranean climate, on the Apulia carbonatic plateau and the full sequence of soil horizons is Ap, 2AB, 2Bt and 2Cr (USDA-NRCS, 2010). Clay skins were clearly visible in the Bt horizon, while the lithological discontinuity was indicated because of the lower amounts of calcareous coarse fragments in deeper horizons than in the surface one. The soil is under cultivation with scattered olive trees, and a minimum


Fig. 1. Location of the study areas and soil profiles surveyed (a) Rutigliano soil profile; (b) Vauda di Nole soil profile.

tillage, consisting in a mowing and chopping weeds that remain on site, is regularly practiced. The Vauda di Nole profile (PR2) develops in a temperate climate on fluvio-glacial terraces from MidPleistocene (Mindel-Riss interglacial) of mixed lithology and has a more complex sequence of soil horizons. In addition to illuvial ones (Btx, fragipans with abundant clay skins), E horizons were also well visible by a change in color and soil consistence. The soil is under natural vegetation: a heathland resulting from the degradation of the mixed oak and hornbeam climax forest. The site characteristics and the general information on the soil profiles are shown in Table 1. In both cases, the Bt meets the requirements for diagnostic argillic horizons and the soils were therefore classified as Xeralf and Udalf, respectively (USDA-NRCS, 2010). 2.2. Soil analyses Soil samples were air-dried and 2-mm sieved before laboratory analyses, that were performed according to the Italian Official Methods of Soil Analysis (Mipaf, 2000). Particle-size distribution was determined by the pipette method after removing carbonates and organic matter. Soil samples were analyzed also for pH (1:2.5, w/v), total organic carbon (Walkley and Black method, Mipaf, 2000), and total nitrogen (Kjeldahl method, Mipaf, 2000). Iron in amorphous oxides was extracted from bulk soil samples with


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Table 1 Soil-forming factors and classification of the two Alfisol profiles studied. Pedon

Sampling date

Sampling point (UTM33-WGS84)

Elevation m asl

Slope (%)

Rainfall (mm)

Temp (°C)

Moisture regimea

Vegetation/Land use



February 02, 2011



Chemical and spectroscopic characteristics of humic acids and dissolved organic matter along two Alfisol profiles.

The aim of this study was to elucidate the heterogeneous structural and functional composition of humic acids (HAs) and dissolved organic matter (DOM)...
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