In this work, the occurrence of the untreated and treated olive mill wastewaters (OMW) amendment on the soil organic matter recycling and on its microbial biomass evolution were investigated. Compared to the control, soils amended with untreated and treated OMW showed high levels of organic and mineral matters. Soil amended with untreated OMW presents low levels of total and inorganic nitrogen (0.38 ± 0.03 and 0.08 ± 0.02 mg g-1 dry soil). Treated OMW had a little content of pollutants (COD = 4 g l-1± 0.4; phenolic compounds = 0.6 g l-1± 0.04) and organic matter brought by these residues was rapidly mineralized in the soil. The number of heterotrophic bacteria was increased (from 54 ± 5 .105 CFU g-1 dry soil in control soil to 123 ± 11. 105 CFU g-1 dry soil) in response of the OMW amendment, mainly following C/N ratio correction. The amendment of the soil with untreated OMW improved the soil carbon content (2.18 times higher) while the specific respiration remained very low. However, the amendment with treated OMW positively affects the soil specific respiration that increases from 6.1 in control soil to 9.75 in soil amended with treated OMW. This phenomenon was accompanied by an enhancement (from 12 ± 2 .104 CFU g-1 dry soil in control soil to 83 ± 5. 104 CFU g-1 dry soil in soil amended with OMW) of nitrifiers number , urease and ammonium oxidases activities.
Key words: Olive mill wastewaters, soil, organic matter, nitrifiers, mineralization.