Hydrolysis of pyrophosphate in a highly calcareous soil: A solid-state phosphorus-31 NMR study

Pyrophosphate is the main form of condensed P in the fluid fertilizer ammonium polyphosphate (APP). When APP is applied to soil, pyrophosphate is hydrolyzed to orthophosphate. Hydrolysis of pyrophosphate was investigated in a highly calcareous soil over 3 wk. Changes in P speciation were measured using solid-state P-31 nuclear magnetic resonance (NMR) spectroscopy for dried soil samples and ion chromatography for NaOH extracts. Both techniques showed a decrease in pyrophosphate and concomitant increase in orthophosphate concentration with time, but the rate or pyrophosphate hydrolysis calculated from the NaOH extract measured by ion chromatography was two times faster than the rate calculated from the NMR data indicating differences in P speciation between methods. The NMR technique employed spin counting to determine the proportion of P nuclei observable by NMR. Since this proportion (71-88%) was greater than the proportion of P extracted by NaOH and detected by ion chromatography (38-71 %), we concluded that the NMR technique was able to quantify more of the pyrophosphate added than the extraction method. The results of the study suggest that solid-state P-31 NMR is a powerful noninvasive technique for the investigation of pyrophosphate reactions in soil and that pyrophosphate has the ability to persist for several weeks in highly calcareous Australian soils in the presence of microbial activity. This has important implications for P availability processes where pyrophosphate is supplied as a fertilizer and is currently under investigation in Australian soil types.