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Fluid Journal : Fluid Journal 2005-2007
F all 2005 Fluid Journal 2 orthophosphate concentration ov er time. According to the NMR data (Figure 1A) the concentration of pyrophosphate decreases to approximately half of the original concentration added and is converted to orthophosphate, presumably by hydrolysis. Soil spectra are obtained by direct polarization using P solid-state NMR. The native P spectra are for soil incubated one week at 75 percent of water holding capacity with no P added. After one day, the spectra show the pyrophosphate added and the distinct spinning side-band pattern that it has. Six spinning side bands are observable throughout the incubation and they remain in the same proportion of the central band for each P species throughout the incubation, suggesting that we were consistently measuring the same species. After one day the peak for orthophosphate is very small and mostly represents the native P. As time progresses the orthophosphate peak grows as pyrophosphate hydrolyzes, and consequently the peak for pyrophosphate decreases. After 21 days of incubation, approximately half of the pyrophosphate peak remains and the orthophosphate peak has increased in size, including both native P and P that has hydrolyzed from pyrophosphate. Due to the success of this technique for the study of hydrolysis of pyrophosphate, further investigations will look at other highly P fixing soil types and the biological and chemical causes of hydrolysis will be studied. The work conducted on hydrolysis using NMR will be complemented by an isotopic double labeling technique currently being developed to investigate the potential availability of P supplied to the soil as pyrophosphate. Sorption Two soils were selected to demonstrate the relationships observed between P in solution and total P sorbed. The two soils selected were: 1) Warramboo of South Australia, a high pH, highly calcareous soil and 2) Hamilton of Victoria, a low pH soil with relatively moderate levels of total Fe and Al. The sorption characteristics of both soils show that a greater amount of total P was sorbed when applied as pyrophosphate as compared to orthophosphate (Figure 2 A and B). The Freundlich isotherm data (Table 1) shows a satisfactory fit for both the Warramboo and Hamilton P sorption data with a R2 of 0.99 to 0.96 for orthophosphate and pyrophosphate, respectively. The Kf values in Table 1 indicate that there is a higher level of retention where pyrophosphate is added to both soils as compared to orthophosphate. Further work in this study investigated the behavior of calcium, iron, and dissolved organic carbon (DOC) in solution (data not shown). The general trends indicated a significant decrease in the concentration of calcium in solution and an increase in the iron and DOC concentration in solution with the addition of pyrophosphate as compared to orthophosphate. The results of this study suggest that the greater performance of P supplied as APP in the field is not due to reduced sorption of pyrophosphate as compared to orthophosphate. Conclusions Pyrophosphate is able to persist in highly P-fixing Australian soil types with approximately half of the P added as pyrophosphate remaining as pyrophosphate after three weeks of incubation. Sorption data suggest that pyrophosphate is rapidly sorbed in these soil types. Work under way, investigating the potential availability of pyrophosphate using a double labeling technique, will further contribute to our understanding of the behavior of polyphosphates, pa rticula rly pyrophosphates in Australian soil types. McBeath is a PhD student, Dr. McLaughlin is senior principal scientist, Dr. Smernik is research Fellow, and Dr. Bunemann is research Fellow, School of Earth and Environmental Sciences, University of Adelaide, South Australia; Dr. Lombi is senior research scientist who also works with Dr. McLaughlin (joint appointment), CSIRO Land and Dr. Holloway is principal research scientist, South Australian Research and De velopment Institute, Minnipa. Table 1. P sorbed (mg kg-1) vs. P in solution (mg L-1) for Warramboo (A) and Hamilton (B) soils. Soil Fertilizer Kf nR2 Hamilton OP 80.83 0.47 0.99 Hamilton PyP 670.89 0.34 0.96 Warramboo OP 52.88 0.52 0.97 Warramboo PyP 671.19 0.34 0.96 OP=orthophosphate PyP=pyrophosphate Table 1. Freundlich sorption parameters for orthophosphate and pyrophosphate in Warramboo and Hamilton soils.
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