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The effectiveness of microbial inoculants with different and interactive metabolic abilities, native arbuscular mycorrhizal (AM) fungi and a plant growth-promoting yeast (PGPY) Yarowia lipolytica, were assayed in a natural semiarid soil. The soil was either amended or was not amended with microbiologically treated residues, sugar beet (SB), or olive processing waste (OPW). Dorycnium pentaphyllum, used for revegetation purposes in semiarid soils, was selected as the test plant. The effects of microbial inoculations and/or OPW and SB amendments were evaluated in terms of plant growth and P nutrition. β-glucosidase and phosphatase activities in the rhizosphere, and other soil characteristics such as aggregate stability, were determined as well. The supply of treated OPW, SB, or both alleviated limiting P conditions in the soil, and inoculations of Y. lipolytica or native AM fungi increased plant P acquisition to the highest extent in residue-amended soils. Autochthonous mycorrhizal populations and native Rhizobium sp. were not able to colonize roots of Dorycnium plants growing in natural soil, but inoculants and the amendments (OPW or SB) applied enhanced the formation of such symbiotic structures. Root AM colonization depended on the AM inoculum applied (enriched population of native AM fungi), but this value was increased considerably by the amendments and by Y. lipolytica inoculation. The low nodulation found in this degraded soil was also increased by OPW and/or SB and by microbial inoculations. The application of organic amendments to the soil increased β -glucosidase and phosphatase activities as well as aggregate stability, water-soluble C, and water-soluble carbohydrates. These values were also altered by the microbial inoculants. Treatments increased the biodiversity of AM populations in this arid soil. Application of biologically treated agrowastes was able to increase soil microbial activity and plant development, which are highly depressed in many stressed areas. Thus, these treatments can be used as a valuable strategy in desertified areas.