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PEDOLOGY

Isotopic Exchangeability, Hydrolysis and Mobilization Reactions of Pyrophosphate in Soil

^a Soil and Land Systems, School of Earth and Environmental Sciences, Univ. of Adelaide, PMB1, Waite Campus, Glen Osmond, SA 5064, Australia
^b CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia
^c current address: Plant and Soil Science Lab., Dep. of Agricultural Sciences, Faculty of Life Sciences, Univ. of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark
^d current address: Institute of Plant Sciences, Swiss Federal Institute of Technology Zurich, Eschikon 33, 8315 Lindau, Switzerland

^* Corresponding author (therese.mcbeath{at}adelaide.edu.au).

Liquid polyphosphate fertilizers have shown significant yield increases compared with granular orthophosphate fertilizer on highly P-fixing soils in Australian cropping trials. Polyphosphate fertilizers contain mixed P species (predominantly pyrophosphate and orthophosphate) and are therefore chemically different from most other inorganic P fertilizers, which contain only orthophosphate. To determine the mechanisms causing the yield responses to polyphosphate fertilizer, a method for investigating the isotopic exchangeability (lability) of P added as pyrophosphate was developed and tested. This method enabled the quantification of the hydrolysis reaction of pyrophosphate to orthophosphate in soil solution. As the rate of pyrophosphate applied increased, the proportion of pyrophosphate added that was isotopically exchangeable increased, and the percentage of pyrophosphate added that hydrolyzed to orthophosphate decreased. The method was further developed to test the hypothesis that pyrophosphate addition to soil may cause the mobilization of native orthophosphate reserves. The results suggested that there was no mobilization of native P after 7 d of incubation in two calcareous soils.

Abbreviations: OP, orthophosphate • PP, pyrophosphate