Quantification of Myo-Inositol Hexakisphosphate in Alkaline Soil Extracts by Solution 31p NMR Spectroscopy and Spectral Deconvolution

Abstract

Inositol phosphates are the dominant class of organic phosphorus (P) compounds in most soils, but they are poorly understood because they are not easily identified in soil extracts. This study reports a relatively simple technique using solution 31P NMR spectroscopy and spectral deconvolution for the quantification of myo-inositol hexakisphosphate (phytic acid), the most abundant soil inositol phosphate, in alkaline soil extracts. An authentic myo-inositol hexakisphosphate standard added to a re-dissolved soil extract gave signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1. Spectral deconvolution quantified these signals accurately (102 ± 4%) in solutions containing a mixture of model P compounds by resolving the envelope of signals in the orthophosphate monoester region. In NaOH-EDTA extracts from a range of lowland permanent pasture soils in England and Wales, concentrations of myoinositol hexakisphosphate determined by spectral deconvolution ranged between 26 and 189 mg P kg-1 soil, equivalent to between 11 and 35% of the extracted organic P. Concentrations were positively correlated with oxalate-extractable aluminum and iron but were not correlated with total carbon, total nitrogen, clay, or the microbial biomass. This suggests that myo-inositol hexakisphosphate accumulates in soils by mechanisms at least partially independent of those controlling organic matter stabilization and dynamics. Furthermore, myo-inositol hexakisphosphate concentrations were positively correlated with plant-available inorganic P and negatively correlated with the carbon-to-organic P ratio, suggesting that biological P availability may, in part, regulate myo-inositol hexakisphosphate concentrations in soils, perhaps because organisms capable of degrading this compound are favored in more P-limited environments. Solution 31P NMR spectroscopy and spectral deconvolution offers a relatively simple method of quantifying myo-inositol hexakisphosphate in soil extracts. (Soil Science 2003;168:000-000)

Identification of inositol hexakisphosphate binding sites in soils by selective extraction and solution 31P NMR spectroscopy

“…In acidic soils, associations with amorphous Al and Fe hydroxides are believed to be the most important mechanism of IP 6 stabilization (Celi and Barberis, 2007). This is supported by correlations between amorphous metals and IP 6 across a wide variety of soils (Anderson et al, 1974;McKercher and Anderson, 1968;Turner et al, 2007Turner et al, , 2003Vincent et al, 2012). However, the relative contribution of these stabilization processes remains poorly understood, in part because most recent studies have extracted IP 6 from soils by a single-step NaOH-EDTA procedure, which is assumed to extract mineral associated IP 6 as well as IP 6 associated with the organic soil matrix (Turner et al, 2005a).…”

“…Several studies have reported correlations between IP 6 and amorphous metal oxides in acidic soils (Anderson et al, 1974;Giaveno et al, 2008;McKercher and Anderson, 1968;Turner et al, 2003). In addition, IP 6 concentrations have been shown to fluctuate in parallel with changes in amorphous Al and Fe hydroxides during long-term pedogenesis .…”

“…From these peak areas, the contribution of the individual P groups was calculated relative to the TP of the extract. Peaks were identified by comparison with literature reports (Turner et al, 2003(Turner et al, , 2005b. Signals in spectra of oxalate extracts were slightly upfield of the corresponding signals in other extracts which may be caused by the high ionic concentration in the oxalate extracts.…”

Organic phosphorus in Madagascan rice soils

“…The spectra are plotted with 1 Hz line broadening. (Turner et al, 2003b). ‡ Determined by integration of the signal at 4.2 ppm (Turner and Richardson, 2004).…”

“…Spectra were plotted with a line broadening of 1 Hz to preserve resolution in the phosphate monoester region. Concentrations of myo-inositol hexakisphosphate were determined by multiplying by six the signal at approximately 5.9 ppm arising from the C-2 phosphate on the inositol ring (Turner et al, 2003b). Quantification of the three other signals from this compound tends to overestimate myo-inositol hexakisphosphate due to interference with signals from other compounds.…”