Production and elimination of water extractable organic matter in a calcareous soil as assessed by UV/Vis absorption and fluorescence spectroscopy of its fractions isolated on XAD-8/4 resins


  • Hassouna Mohammad
  • Théraulaz Frédéric
  • Massiani Catherine

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Water extractable organic matter (WEOM) in soil refers to a broad mixture of water-soluble structures recoverable in laboratory through predetermined procedures. In this work, we investigate the mechanisms controlling the occurrence of WEOM fractions isolated on XAD-8/4 resins in a calcareous soil profile as a function of soil depth and seasonal climatic changes. WEOM was roughly collected on a bi-weekly basis during eight consecutive months from an experimental field in the Mediterranean region, south-east of France. Each fraction was analysed for organic carbon content and further characterised using UV/Vis absorbance and 3D fluorescence spectroscopy. The concentrations of hydrophobic (HPOA), transphilic (TPIA) and hydrophilic (HPIA) acids fractions isolated from WEOM followed similar decreasing trends with increasing soil depth, but HPOA decreased most sharply. Specific UV absorbance at 254 nm (SUVA(254)) indicated that HPOA's distribution along the soil column is strongly influenced by the selective removal of aromatic structures, which could be controlled by adsorption mechanisms involving organo-mineral interactions and/or by a shift towards less aromatic precursors in deeper soil layers. 3D spectra of fluorescence revealed the presence of four types of fluorophores: two humic-like (A and C) common to all fractions, one lignin-like (H) specific to HPOA particularly in the upper soil layer, and two protein-like (B1 and B2) specific to TPIA. The importance of the humic component in all fractions was verified by the highly significant correlations between their carbon content and the intensity of humic-like peaks. The aging index (IA/IC) and the fluorescence index (FI), two indicators derived from 3D fluorescence spectra, showed an increase in the microbial input and the degree of humification in HPOA as a function of increasing soil depth. By comparison, IA/IC and FI values in HPIA and TPIA indicated that both fractions were composed almost exclusively of microbial-derived structures, regardless of soil depth. Principal component analysis (PCA) showed the association of temperature (T) to the carbon content in all fractions extracted from the soil layers 0-30 and 30-60 cm, suggesting possible role of microbial activity on their occurrence. The hydraulic regime in soil, controlled by the frequency of rainfall events, affected most significantly HPIA across the entire soil column. PCA, however, failed to identify any consistent trends relating the qualitative properties of WEOM fractions to climatic variables. (C) 2012 Elsevier B.V. All rights reserved.

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