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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Pseudomonas cepacia–Mediated Rock Phosphate Solubilization in Kaolinite and Montmorillonite Suspensions

^a Agricultural Research Organization, Inst. of Soil, Water, and Environmental Sciences, Bet Dagan 50250, Israel
^b Idaho National Engineering and Environmental Lab., Biotechnology Unit, Idaho Falls, ID 83415 USA

vwbysf{at}agri.gov.il

Pseudomonas cepacia is known as a rock phosphate (RP) solubilizer in bioreactors and in soils. The objectives of this study were to determine the production rates of gluconic acid (GA, pK_d 3.41) and 2-ketogluconic acid (KGA, pK_d 2.66) by the bacteria in the presence of clay minerals which prevail in soils, and the resulting rate and extent of orthophosphate (OP) release into the suspension solutions. Suspensions (1:40) of RP, RP + Ca–kaolinite (CaKL), RP + Ca–montmorillonite (CaMT), and RP + K–montmorillonite (KMT) were inoculated with P. cepacia E37. The electrical conductivity (EC) and pH, and the OP, glucose, GA, KGA, Ca, and Al concentrations were determined in the suspension solutions as functions of time. In a given clay system, the rate-limiting step in RP dissolution was the rate of GA release by the E37. This acid lowered the pH of all the clay suspensions to 2.7 to 2.8, which resulted in a pronounced increase in the OP concentration in solution, Cp. As glucose was depleted from the system, the KGA concentration increased with a concomitant lowering in pH to 2.5. At this pH, a sharp decline in Cp occurred, which was attributed to Al release by the alumosilicates, and formation of a new, stable Al–P or Fe–P solid phase. The E37 glucose oxidation efficiency (GOE) was considerably inhibited in CaKL as compared with CaMT or KMT. The GA and KGA adsorption by the clays or their Ca complexation did not play a role in the E37-mediated RP solubilization.

Abbreviations: CaKL, Ca–kaolinite • CaMT, Ca–montmorillonite • EC, electrical conductivity • GA, gluconic acid • GOE, glucose oxidation efficiency • IAP, ion activity product • KGA, 2-ketogluconic acid • KKL, K–kaolinite • KMT, K–montmorillonite • OP, orthophosphate • RP, rock phosphate