Use of olive mill wastewater as an organic amendment for Mentha spicata L.

In this study, we investigated the impact of olive mill wastewater (OMW) spreading on soil characteristics, Mentha spicata L. yields and essential oil composition. Soil characterization and spearmint cuttings plantation were done after two months from OMW spreading at 8 l m−2 and 16 l m−2. OMW spreading at 8 l m−2 induced a significant increase in abundances of yeasts, actinomycetes, nitrogen aerobes and cellulolitics in soil while the increase was smaller or insignificant for 16 l m−2. Moulds showed no changes in abundances for the two doses while ammonifiers presented a deep decrease more important for the higher dose. Soil phenols joined the control for the dose 8 l m−2 and stayed significantly higher for the dose 16 l m−2. The soil humification degree was significantly increased for the two doses with higher increase for 8 l m−2. Concerning plant response, OMW spreading at 8 l m−2 allowed a significant increase of 94% of shoots weight, 15% of shoots size, 145% of number of floral spikes, 60% of stolons weight, 51% of stolons size and no significant increase in the number of produced stolons per plant. The essential oil yield was significantly increased of 17% in comparison with control. For the dose 16 l m−2, there were no increase for all the parameters excepted the size of stolons. The essential oil yield was reduced of 52%. The essential oil composition did not show important changes when the amount spread was 8 l m−2. However, for 16 l m−2, we obtained a disappearance of β-pinene, limonene and cis-jasmone, major compounds of the essential oil and appearance of some compounds newly synthesized. An increase of concentration was obtained for dihydrocarvone, dihydrocarvyl acetate, β-bourbonene, caryophyllene, germacrene D and cis-calamenene.