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Journal of Soils and Sediments

, Volume 18, Issue 4, pp 1303–1313 | Cite as

Selective effects of forest fires on the structural domains of soil humic acids as shown by dipolar dephasing 13C NMR and graphical-statistical analysis of pyrolysis compounds

  • Gonzalo Almendros
  • Pilar Tinoco
  • José-María De la Rosa
  • Heike Knicker
  • José-Antonio González-Pérez
  • Francisco J. González-Vila
Natural Organic Matter: Chemistry, Function and Fate in the Environment

Abstract

Purpose

Data management strategies of pyrolysis results and NMR acquisition modes were examined in humic acids (HAs) from control soils and fire-affected soils. The information supplied by dipolar dephasing (DD) 13C NMR spectroscopy and Curie-point pyrolysis were used to assess chemical structures hardly recognizable and measurable, or of unclear interpretation, when using 13C NMR under standard acquisition pulses (cross-polarization/magic angle spinning, CPMAS).

Materials and methods

The HAs were isolated from two forest soils under Pinus halepensis and Pinus sylvestris in control and burned sites affected by medium or severe-intensity wildfires. For NMR analyses, during DD acquisition conditions, a 180° 13C pulse was inserted to minimize phase shifts. Curie-Point pyrolysis was carried out at 510 °C for 5 s, and the pyrolysis fragments were analyzed by GC/MS. The total abundances of the major pyrolysis products were compared by an update of the classical Van Krevelen’s graphical-statistical approach, i.e., as surface density values in the space defined by the compound-specific H/C and O/C atomic ratios.

Results and discussion

The DD 13C NMR experiments displayed significant differences in the HA spectral profiles as regards to the standard CPMAS 13C NMR acquisition conditions, mainly in the chemical shift region of alkyl structures as well as for tannin- or carbohydrate-like O-alkyl structures. In fact, the comparison between DD and CPMAS solid-state NMR suggested shortening of alkyl chains and generation of carbohydrate-derived, unsaturated structures—viz. furans—which adds to the aromatic domain. Pyrolytic results showed fire-induced specific changes in HAs chemical structure and its molecular diversity. The changes were evident in the location and sizes of the different clusters of pyrolysis compounds defined by their atomic ratios.

Conclusions

The DD 13C NMR provided specific information on the fate of aliphatic structures and the origin of unsaturated HA structures, which could be helpful in differentiating “inherited” from “pyrogenic” aromatic structures. This is further confirmed by the analysis of the molecular assemblages of pyrolytic products, which showed accumulation of condensed polyaromatic domains in the HAs after the high-intensity fire, accompanied by a recalcitrant alkyl hydrocarbon domain. Medium-intensity fire led to aromaticity increase due to a selective accumulation of lignin-derived phenols concomitant to the depletion of aliphatic hydrocarbon constituents.

Keywords

Curie-point pyrolysis Dipolar dephasing Forest fires damage levels Humic acid structural changes NMR 

Notes

Acknowledgements

The contributions by three anonymous reviewers and the financial support by the Spanish CICyT (grant CGL2013-43845-P) are gratefully acknowledged.

Supplementary material

11368_2016_1595_MOESM1_ESM.pdf (534 kb)
ESM 1 (PDF 533 kb)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  • Gonzalo Almendros
    • 1
  • Pilar Tinoco
    • 2
  • José-María De la Rosa
    • 3
  • Heike Knicker
    • 3
  • José-Antonio González-Pérez
    • 3
  • Francisco J. González-Vila
    • 3
  1. 1.MNCN (CSIC)MadridSpain
  2. 2.University Alfonso X el SabioMadridSpain
  3. 3.Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC)SevilleSpain

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