Direct evidence for the mineralization of inositol phosphates in soil as revealed by 31P NMR analysis of soil suspensions treated with phytases and organic anions

Soil suspension assays were used to demonstrate synergistic interaction between citrate and phytase for mobilization and in situ mineralization of organic P. This synergy was evident in both unfertilized and fertilized soil where P mobilization by citrate was increased by 3.7 to 7.0-fold across both inorganic and organic pools. In the presence of citrate, phytase from different fungal sources further increased the proportion of P mineralization by an average of 47% from the total extracted P. Specific analysis by 31P NMR of the monoester-P extracted from the fertilized soil showed that the P released was associated with both direct hydrolysis of identifiable biomolecules of inositol hexakisphosphate (IHP, identified as the myo and scyllo isomers) and via a significant reduction of the broad-peak of monoester P. Functionality of endogenous soil phosphatase and phytase activities were also evident in the mineralization assays. The specific hydrolysis of IHP was further enhanced when phytase was added, where the total concentration of the extracted IHP was reduced by 92%. The addition of phytase in the absence of citrate (i.e., water suspensions only) by contrast had no measurable effect on the mineralization of soil organic P. Our results further our understanding of the biochemical nature and speciation of organic P in soils and the potential contribution that mineralization of organic P makes to the soil P cycle. This is particularly relevant to the rhizosphere where functional interaction of citrate and phytase and utilization of specific organic P substrates may offer opportunity for improved P-use efficiency.