Original article Changes in biological activity of a degraded Mediterranean soil after using microbially-treated dry olive cake as a biosolid amendment and arbuscular mycorrhizal fungi M.M. Alguacil a , F. Caravaca a, *, R. Azco ´n b , A. Rolda ´n a a Department of Soil and Water Conservation, CSICdCentro de Edafologı´a y Biologı´a Aplicada del Segura, P.O. Box 164, Campus de Espinardo, 30100 Murcia, Spain b Microbiology Department, CSICdEstacio ´n Experimental del Zaidı´n, Profesor Albareda 1, 18008 Granada, Spain article info Article history: Received 11 October 2007 Accepted 11 February 2008 Published online 3 March 2008 Keywords: Arbuscular mycorrhizal fungi Dorycnium pentaphyllum Dry olive cake Enzyme activities Aggregate stability Semiarid areas abstract A field experiment was carried out to assess the effect of a combined treatment involving addition of Aspergillus niger-treated dry olive cake (DryOC) in the presence of rock phosphate, plus pre-transplant inoculation of seedlings with the arbuscular mycorrhizal (AM) fungi Glomus intraradices, Glomus deserticola or Glomus mosseae, on the establishment of Dorycnium pentaphyllum L., in a degraded semiarid Mediterranean area. Associated changes in soil labile C fractions, enzyme activities and aggregate stability were also observed. One year after planting, the combined treatment of fermented DryOC addition and inoculation with AM fungi, particularly with G. mosseae (on average 328% greater than control plants), had the strongest effect on the shoot biomass of D. pentaphyllum. Only the fermented DryOC addition increased assimilable P, total N and aggregate stability, the greatest increase being in the soil available P content (about four-fold higher than in the non-amended soil). Both the addition of fermented DryOC and the mycorrhizal inoculation treatments significantly increased enzyme activities of rhizosphere soil (dehydrogenase, protease-BAA, acid phosphatase and b-glucosidase). The microbially-treated DryOC proved to be an effective amendment for improving the soil quality which, in turn, enhanced the success of revegetation with mycorrhizal D. pentaphyllum seedlings. ª 2008 Elsevier Masson SAS. All rights reserved. 1. Introduction The main areas of olive oil production are concentrated in the Mediterranean Basin countries, Spain being the greatest pro- ducer (about 990,400 tons annually). The main system of olive oil extraction used in Spain (two-phase centrifugation system) generates large quantities of a solid residue called dry olive cake (DryOC) or ‘‘alperujo’’. The agronomic utilisation of DryOC has increased steadily in recent years as an alternative nutrient and organic matter source, and as an acceptable method for its disposal. However, the use of DryOC has been seen to have a detrimental effect on seed germination, plant growth and microbial activity [17]. In fact, several studies have reported phytotoxic and antimicrobial effects of olive- mill residues due to the phenol, and organic acid contents [16,17]. Efforts to decrease the environmental impact of * Corresponding author. Tel.: þ34 968 396337; fax: þ34 968 396213. E-mail address: fcb@cebas.csic.es (F. Caravaca). available at www.sciencedirect.com journal homepage: http://www.elsevier.com/locate/ejsobi 1164-5563/$ – see front matter ª 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2008.02.001 european journal of soil biology 44 (2008) 347–354