Tillage Long-Term Effects on Soil Organic Matter Humification and Humic Acids Structural Changes in Regosol Profiles Typical of an Arid Region

This study aims to assess soil organic matter (SOM) humification and changes in molecular structures of humic acids (HAs) induced by long term management practices over soil profile under arid climate in south-eastern Tunisia. Two experimental fields were studied: the first was cultivated with olive tree and tilled for long term (CT) while the second was uncultivated without any management practices and with native vegetation (NC). Soil samples were collected at different depths from (CT) and (NC) profiles to determine chemical properties. Humic acids were isolated and studied by measuring UV-visible ratios (E₄₆₅/₆₆₅, E₂₈₀/₆₆₄ and E₄₇₂/₆₆₄) and fluorescence spectroscopy analysis. Results showed that long-term tillage practices reduced the SOM amount, aromatic condensation, molecular size and humification degree in surface layer of CT confirmed by the decrease in the fluorescence emission areas and UV-visible ratios. Inversely, in deep layers, results showed a greatly humified organic matter and aromatic structures condensation in CT. Long-term tillage leads to a significant decrease in aromatic condensation and OM humification in surface layers. Unmanaged soils provided a favorable environment to maintain OM chemical structures and restore soil quality under long-term management practices.