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This study attempted to demonstrate that biological residue from a new biological wastewater treatment is a resource for improving quality of arid soils and plant growth. After tertiary wastewater treatment, debris composed of alginate beads containing the microalgae Chlorella sorokiniana and the plant growth-promoting bacterium Azospirillum brasilense was used as an amendment for eroded, infertile desert soil having low levels of organic matter. A. brasilense survived in these used dried beads for at least one year. Three consecutive applications of the dry debris increased organic matter, organic carbon, and microbial carbon in the soil. Growth of sorghum in the amended soil was greater than plants grown in low organic matter, untreated soil or soil amended with beads containing other combinations of alginate, microalgae, or bacteria. The surface of plant roots growing in the amended soil was heavily colonized by A. brasilense, with no endophytic colonization; root tips were the preferred sites of colonization. (C) 2011 Published by Elsevier B.V.