Effects of amendment with olive mill by-products on soils revealed by nitrifying bacteria.

We investigated the effect of olive mill by products amendment on soil characteristics and on nitrifying and heterotrophic bacteria content, in comparison with not amended soils. The effect of the amendment on organic carbon content was also evidencied. No differences were pointed out in terms of heterotrophic bacteria, whereas addition of olive mills by products increase nitrifying bacteria content in soils with addition of organic mixtures containing olive mill wastes. Two nitrifying bacterial strains were isolated from amended soils named ISAFOM-B3 and ISAFOM-C2, and 16S rDNA gene sequencing assigned them to the genus Arthrobacter and to the ?-Proteobacteria subclass respectively. A higher nitrate content was pointed out in enrichment cultures arranged with amended soils compared to non-amended ones (0.98 mg g-1 d.w. and 0.18 mg g-1 d.w. respectively). Nitrifying bacteria were imaged by fluorescent in situ hybridization. A high total organic carbon content (e.g. 1.87 %) was detected in the amended soils, with an improvement of the humification indexes. This study suggest a positive effect of the addition of olive mill wastes by products on soils.