ORIGINAL ARTICLE Comparing phosphorus mobilization strategies using Aspergillus niger for the mineral dissolution of three phosphate rocks K.D. Schneider 1 , P. van Straaten 1 , R. Mira de Ordun ˜a 2 , S. Glasauer 1 , J. Trevors 3 , D. Fallow 1 and P.S. Smith 1 1 Department of Land Resource Science, University of Guelph, Guelph, ON, Canada 2 Department of Food Science and Technology, NYSAES, Cornell University, Geneva, NY, USA 3 Department of Environmental Biology, University of Guelph, Guelph, ON, Canada Introduction Phosphorus (P) deficiencies are limiting crop production in many agricultural soils worldwide. The use of synthetic water-soluble P fertilizers is limited in developing coun- tries because of the significant product and associated transportation costs (Sagoe et al. 1998; Van Kauwenbergh 2006). In addition, chemical P fertilizers are prohibited for use by organic farming standards. As a result, locally available sources of raw phosphate rock (PR) have been recognized as alternative P fertilizers (Arcand and Schnei- der 2006; Van Straaten 2007). PR from different sources can vary greatly in agro- nomic effectiveness. In general, PR solubility, which has been found to correlate with crop response, increases as carbonate substitution for phosphate in apatite (e.g. Ca 10 [(PO 4 ) 6 ) x (CO 3 ) x ](F 2+ x ) (Slansky 1986), the main P-containing mineral in PR, increases (Chien and Menon 1995; Van Kauwenbergh 2006). The P released from directly applied milled PR is often too low to supply sufficient P for crop uptake, even when more soluble sedimentary PRs, with greater carbonate substitution, are used (Vassilev et al. 2001; Van Straaten 2007). The disso- lution of PR can be increased through the addition of H + ions according to the following simplified equation shown (adapted from Sagoe et al. 1998): Ca 10 ðPO 4 Þ 6 F 2 þ 12H þ $ 10Ca 2þ þ 6H 2 PO  4 þ 2F  ð1Þ The ability of H + ions to accelerate the dissolution of PR is basis for the production of synthetic P fertilizers, where Keywords Aspergillus niger, citric acid, organic acids, oxalic acid, phosphate rock, phosphorus availability, solubilization. Correspondence Kim D. Schneider, Department of Land Resource Science, University of Guelph, Guelph, ON, Canada N1G 2W1. E-mail: kschne01@uoguelph.ca Present address R.M. de Ordun ˜ a, Department of Food Science & Technology, New York State Agricultural Experiment Station, Food Research Laboratory, 630 W. North Street Geneva, NY 14456-1371, USA. 2009/0404: received 3 March 2009, revised 24 April 2009 and accepted 2 July 2009 doi:10.1111/j.1365-2672.2009.04489.x Abstract Phosphorus deficiencies are limiting crop production in agricultural soils worldwide. Locally available sources of raw phosphate rock (PR) are being recognized for their potential role in soil fertility improvement. Phosphorus bioavailability is essential for the efficiency of PRs and can be increased by acid treatments. The utilization of organic acid producing micro-organisms, notably Aspergillus niger, presents a sustainable alternative to the use of strong inor- ganic acids, but acid production of A. niger strongly depends on the mineral content of the growth media. This study compared the phosphorus mobiliza- tion efficiency of two biological treatments, namely addition of acidic cell-free supernatants from A. niger cultivations to PRs and the direct cultivation of A. niger with PRs. The results show that addition of PR to cultivations leads to significant differences in the profile of organic acids produced by A. niger. Additions of PR, especially igneous rocks containing high amounts of iron and manganese, lead to reduced citric acid concentrations. In spite of these differ- ences, phosphorus mobilization was similar between treatments, suggesting that the simpler direct cultivation method was not inferior. In addition to citric acid, it is suggested that oxalic acid contributes to PR solubilization in direct cultivations with A. niger, which would benefit farmers in developing countries where conventional fertilizers are not adequately accessible. Journal of Applied Microbiology ISSN 1364-5072 366 Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 108 (2010) 366–374 ª 2009 The Authors