Abstract
Abstract. Inositol phosphates (IPs) are a major pool of identifiable organic phosphorus (P) in soil. However, insight into their distribution and cycling in soil remains limited, particularly of lower-order IP (IP5 and IP4). This is because the quantification of lower-order IP typically requires a series of chemical extractions, including hypobromite oxidation to isolate IP, followed by chromatographic separation. Here, for the first time, we identify the chemical nature of organic P in four soil extracts following hypobromite oxidation using solution 31P NMR spectroscopy and transverse relaxation (T2) experiments. Soil samples analysed include A horizons from a Ferralsol (Colombia), a Cambisol and a Gleysol from Switzerland, and a Cambisol from Germany. Solution 31P nuclear magnetic resonance (NMR) spectra of the phosphomonoester region in soil extracts following hypobromite oxidation revealed an increase in the number of sharp signals (up to 70) and an on average 2-fold decrease in the concentration of the broad signal compared to the untreated soil extracts. We identified the presence of four stereoisomers of IP6, four stereoisomers of IP5, and scyllo-IP4. We also identified for the first time two isomers of myo-IP5 in soil extracts: myo-(1,2,4,5,6)-IP5 and myo-(1,3,4,5,6)-IP5. Concentrations of total IP ranged from 1.4 to 159.3 mg P per kg soil across all soils, of which between 9 % and 50 % were comprised of lower-order IP. Furthermore, we found that the T2 times, which are considered to be inversely related to the tumbling of a molecule in solution and hence its molecular size, were significantly shorter for the underlying broad signal compared to for the sharp signals (IP6) in soil extracts following hypobromite oxidation. In summary, we demonstrate the presence of a plethora of organic P compounds in soil extracts, largely attributed to IPs of various orders, and provide new insight into the chemical stability of complex forms of organic P associated with soil organic matter.
Highlights
Inositol phosphates (IPs) are found widely in nature and are important for cellular functions in living organisms
Concentrations of total P in NaOH-EDTA soil extracts following hypobromite oxidation ranged from 77 to 578 mg P per kg soil (Table 3), which accounted for 31 % to 48 % of the total P originally present in the extracts
The NaOH-EDTA extraction technique is considered to be a measure of total organic P in soil, which can be subsequently characterised by solution 31P nuclear magnetic resonance (NMR) spectroscopy (Cade-Menun and Preston, 1996)
Summary
Inositol phosphates (IPs) are found widely in nature and are important for cellular functions in living organisms. They are found in eukaryotic cells where they operate in ionregulation processes, as signalling or P storage compounds (Irvine and Schell, 2001). An important IP found in nature is myo-IP6, which is used as a P storage compound in plant seeds Another important species of IP is that of myo(1,3,4,5,6)-IP5, which is present in most eukaryotic cells at concentrations ranging from 15 to 50 μM (Riley et al, 2006). Species of IP1−3 are present in phospholipids such as phosphatidylinositol diphosphates and are an essential structural component of the cell membrane system (Strickland, 1973; Cosgrove and Irving, 1980)
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