BIOAVAILABILITY OF ORGANIC AND INORGANIC PHOSPHATES ADSORBED ON SHORT-RANGE ORDERED ALUMINUM PRECIPITATE

A nonreductive community-level study of P availability was conducted u sing various forms of adsorbed P. Orthophosphate (Pi), inositol hexaph osphate (IHP), and glucose 6-phosphate (G6P) were adsorbed to a short- range ordered Al precipitate. These bound phosphates provided a P sour ce sufficient to support the growth of microbial communities from acid ic Brazilian soils (oxisols). Adsorbed IHP, the most abundant form of organic phosphate in most soils, had the lowest bioavailability among the three phosphates studied. Adsorbed G6P and Pi were almost equally available. The amount of adsorbed Pi (1 cmol P kg(-1)) required to sup port microbial growth was at least 30 times less than that of IHP (30 cmol P kg(-1)). With increased surface coverage, adsorbed IHP became m ore bioavailable. This availability was attributed to a change in the structure of surface complexes and presumably resulted from the decrea sed number of high-affinity surface sites remaining at high levels of coverage. It thus appears that the bioavailability of various forms of adsorbed phosphate was determined primarily by the stability of the p hosphate-surface complexes that they formed, rather than by the total amount of phosphate adsorbed. IHP, having the potential to form stable multiple-ring complexes, had the highest surface affinity and the low est bioavailability. Bioaggregates consisting of bacteria and Al preci pitate were observed and may be necessary for effective release of ads orbed P. Bacteria in the genera Enterobacter and Pseudomonas were the predominate organisms selected during these P-limited enrichments.