Effects of forest fires on water extractable organic matter and humic substances from Mediterranean soils: UV–vis and fluorescence spectroscopy approaches

Humic substances (HS) and water extractable organic matter (WEOM) represent an important part of soil organic matter (SOM) which plays many roles in this ecosystem. Forest fires are known to have potential impacts on soil organic matter. We studied these impacts, first on global HS and the three relevant fractions (humic acids (HA), fulvic acids (FA) and non-humified fraction (NHF)) and secondly on WEOM and the three corresponding fractions (hydrophilic (Hpi), transphilic (Tpi) and hydrophobic (Hpo)) fractionated by the use of XAD-8 and XAD-4 resins. Measurements of total organic carbon (TOC) and the use of both UV–vis and fluorescence spectroscopy gave us quantitative and qualitative results to evaluate the impacts of forest fires abundance and recurrence on the restoration process of the burned soils. As for the TOC-SOM, the quantitative impact of fires on TOC-WEOM is indirect and depends on the fire number. The fires have a direct impact on WEOM quality where, depending on the fraction properties, fires lead to the homogenization of the different compounds of the WEOM. Fires have a direct impact on HS quantity leading to a significant decrease of the amounts. The quality of HS is also affected by fires showing an increase in aromaticity of humified fractions for long time spans (more than 16 years). On a short term the non-humified fractions appear less structured and complex. In general, the more structured the compounds in the fraction, the greater recovery time.