Microbial solubilization of rock phosphate on media containing agro-industrial wastes and effect of the resulting products on plant growth and P uptake N. Vassilev 1, *, A. Medina 2 , R. Azcon 2 & M. Vassileva 1 1 Department of Chemical Engineering, Faculty of Sciences, University of Granada, c/Fuentenueva s/n, Granada, 18071, Spain. 2 Estacion Experimental del Zaidin, Granada, Spain. Corresponding author* Received: 31 May 2006 Key words: agro-industrial wastes, fermentations, fungal microorganisms, plant growth, rock phosphate solubilization Abstract Four agro-industrial wastes were assayed as substrates for microbial solubilization of rock phosphate (RP). Sugar beet wastes (SB), olive cake (OC) and olive mill wastewaters (OMWW) were treated by Aspergillus niger, and dry olive cake (DOC) was treated by Phanerochaete chrysosporium. In conditions of solid-state fermentation 46% of SB and 21% of OC were mineralized by A. niger while 16% of DOC was mineralized by P. chrysosporium. Repeated-batch mode of fermentation was employed for treatment of OMWW by immobilized A. niger, which resulted in conversion of 80% of the fermentable sugars. Acidification of all media treated by A. niger was registered with a simultaneous solubilization of 59.7% (SB), 42.6% (OC), and 36.4% (OMWW) of the total P present in the RP. The same mechanism of RP solubilization was observed in DOC-based medium inoculated with P. chrysosporium but other mechanisms were probably involved during the process. A series of microcosm experiments were then performed in the greenhouse to evaluate the effectiveness of the resulting fermented products. All amendments improved plant growth and P acquisition, which were further enhanced by mycorrhizal inoculation. The level of all studied parameters including the root mycorrhizal colonization depended on the substrate characteristics. The reported biotechnological schemes offer a potential application particularly for degraded soils. Introduction Although phosphorus (P) is quite abundant in many soils, it is one of the major plant nutrients limiting plant growth. P is added to soil in the form of phosphate fertilizers, part of, which is utilized by plants but another part rapidly forms insoluble complexes with soil constituents, thus lowering the overall P use efficiency. Therefore, frequent application of soluble forms of inor- ganic P is needed. However, in practice, as the capacity of soil to bind P is limited, many soils receive P in excess of crop requirements which results in its leaching to the ground water. The runoff from P-loaded soil is accepted as the main factor in eutrophication of natural water reser- voirs (Del Campillo et al., 1999). In view of envi- ronmental concerns and current developments in sustainability, research efforts are concentrated on elaboration of agro-techniques that involve the use of less expensive, though less bioavail- able, sources of plant nutrients such as rock phosphate (RP). It is accepted that there is no substitute of RP as a source of P. However, and particularly for non-acidic soils, a minimum processing is required before application. Even when the soil * Fax: +34-958-248992 E-mail: nbvass@yahoo.com Plant and Soil (2006) 287:77–84 Ó Springer 2006 DOI 10.1007/s11104-006-9054-y