Conformational behaviour of humic substances at different depths along aprofile of a Lithosol under loblolly (Pinus taeda) plantation

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Abstract

The conformation of natural organic matter (NOM) plays a key role in many physical and chemical processesincluding interactions with organic and inorganic pollutants and soil aggregates stability thus directly influencingsoil quality. NOM conformation can be studied by solid state NMR spectroscopy with cross polarization andmagic angle spinning (CPMAS NMR). In the present study we applied CPMAS 13C NMR spectroscopy on threehumic acid fractions (HA) each extracted from a different horizon in a Lithosol profile under Pinus taeda. Resultsshowed that the most superficial HA was also the most aliphatic in character. Amount of aromatic moieties andhydrophilic HA constituents increased along the profile. Cross polarization (TCH) and longitudinal relaxationprotons times in the rotating frame (T1rho(H)) were measured and compared only for the NMR signals generatedby carboxyls and alkyls. This because the signal intensity for the aromatic, C-O and C-N systems was very low,thereby preventing suitable evaluation of TCH and T1rho(H) values for such systems. The cross polarization timesof carboxyls decreased, whereas those of the alkyl moieties increased with depth. Conversely, T1rho(H) valuesincreased for both COOH and alkyl groups along the profile.Polarization transfer from protons to carbons is affected by the dipolar interactions among the nuclei. The strongerthe H-C dipolar interaction, the faster is the rate of the energy exchange. All the factors affecting the dipolarinteraction strength also influence the rate of magnetization transfer. Among the others, fast molecular tumblingand poor proton density around the carbons are responsible for long TCH values. Molecular tumbling and protondensity also affect T1rho(H) values. Namely, the larger the molecular tumbling and the proton density, the fasteris the proton longitudinal relaxation rate in the rotating frame (shorter T1rho(H) values).The decrease of TCH values of COOH groups along the profile was attributed to an increased rigidity ofthe carboxyl systems. Very likely COOH groups may form hydrogen bondings with other hydrophilic HAcomponents that were progressively revealed at deeper depths. On the other hand, increasing of TCH values ofalkyl components was explained with a progressive enhancement of branched chains number. In fact, branchesmay favor molecular flexibility, thereby enabling faster molecular tumbling and longer cross polarization times.Since the amount of branched chains in the alkyl moieties appeared to increase from the top to the bottom of thesoil horizons, the amount of poorly protonated carbons placed in the branch nodes also increases with soil depth.For this reason, proton spin diffusion becomes more difficult and T1rho(H) values increase with the soil depth.Reduced protonation degree may also account for increasing T1rho(H) values of COOH groups.
Lingua originaleEnglish
Numero di pagine0
Stato di pubblicazionePublished - 2009

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