Hydrolysis of Pyrophosphate in a Highly Calcareous Soil: A Solid-State Phosphorus-31 NMR Study

Pyrophosphate is themain form of condensedP 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 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 of 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 techniquewas able to quantifymore of the pyrophosphate added than the extractionmethod. The results of the study suggest that solid-state P 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. CALCAREOUS SOILS INHIBIT the efficiency of P fertilizers due to rapid fixation reactions between the P applied and indigenous Ca present in the soil (Lindsay, 1979; Hedley and McLaughlin, 2005). In South Australia, Eyre Peninsula is a region of highly calcareous soils, which contributes to approximately 40% of the state’s grain production. The gray calcareous soils of this region consist of 19 to 87% calcium carbonate (w/w) (Bertrand et al., 2003). The inefficiency of current P fertilization strategies has stimulated the testing of polyphosphates and other liquid fertilizers in these soils. Fluid P fertilizers account for approximately 23% of America’s and 10% of Canada’s total P fertilizer tonnage (Jennings, 2003). The most widely used fluid P fertilizer in theUSA isAPP (Mortvedt et al., 1999; Jennings, 2003). Polyphosphate fertilizers have been widely used in broadacre agriculture in other parts of the world, but their introduction to Australian agriculture is very recent. Preliminary evidence suggests that polyphosphates may offer substantial benefits over traditional granular P fertilizers in SouthAustralian calcareous soils. For example, in field trials on Eyre Peninsula in 2002, APP provided 36% greater wheat grain yield than granular P fertilizer applied at an equivalent rate of 8 kg P ha on a red calcareous soil (Holloway et al., 2002). At the point of sale, approximately 30 to 40% of the fertilizer P in commercial APP sold in Australia is present as orthophosphate (OP), 50 to 55% is present as pyrophosphate (PP) and the remainder exists as tripolyphosphate and more condensed forms of P (data not shown). Hydrolysis reactions of polyphosphate fertilizer in soil convert more condensed P species (two or more OP groups linked by oxygen bridges) to less condensed forms of P (Lindsay, 1979; Dick and Tabatabai, 1986). Previous data indicates that pyrophosphate constitutes 70 to 90% of the polyphosphate in APP (Khasawneh et al., 1974). Therefore the most common hydrolysis reaction of polyphosphate fertilizer in soil is the conversion of pyrophosphate to orthophosphate (Sutton and Larsen, 1964; Hashimoto et al., 1969). The hydrolysis reaction of pyrophosphate is shown below in Eq. [1]: P2O 7 1 H2O ! 2HPO 4 [1] Several studies have been conducted to compare hydrolysis of pyrophosphate on a range of noncalcareous soil types (Sutton et al., 1966; Gilliam and Sample, 1968; Khasawneh et al., 1979; Sample et al., 1979; Parent et al., 1985; Ahmad and Kelso, 2001). Most methods previously used to assess the behavior of pyrophosphate in soils involve extraction of soil by aqueous media and subsequent determination of orthophosphate-P by colorimetry. Following orthophosphate determination, pyrophosphate-P concentration is measured as the difference between orthophosphate-P and total P determined by acid digestion and colorimetric measurement. These extraction techniques often either ignore extraction efficiency, or assume that the extraction technique is 100% efficient for solid-phase hydrolyzed and nonhydrolyzed pyrophosphate in the soil. Furthermore, the approach of measuring polyphosphate-P concentration by difference ignores the organic P concentration, effectively calculating the polyphosphate-P concentration as polyphosphate-P plus organic P. Here we describe the use of solid-state P NMR spectroscopy for investigating pyrophosphate hydrolysis in a highly calcareous soil. We compared solid-state P NMR T.M. McBeath, R.J. Smernik, and M.J. McLaughlin, Soil and Land Systems, School of Earth and Environmental Sciences, The Univ. of Adelaide, PMB 1, Waite Campus, Glen Osmond, SA 5064, Australia; E. Lombi and M.J. McLaughlin, CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia. Received 9 June 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:856–862 (2006). Nutrient Management & Soil & Plant Analysis doi:10.2136/sssaj2005.0184 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CP, cross polarization; DP, direct polarization; IC, ion chromatography; ICP–AES, inductively coupled plasma atomic emission spectroscopy; NMR, nuclear magnetic resonance; Pobs, percentage of sample P observable by spin counting using P NMR; SSB, spinning side band. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 856 Published online March 29, 2006