Abstract:

Liquid phosphorus-31 (³¹P) nuclear magnetic resonance (NMR) spectroscopy is one of the major analytical technique for the characterization of soil organic phosphorus (P_o) species at the molecular level, and usually requires to extract soil P_ousing NaOH-EDTA (ethylene diamine tetraacetic acid) solution before the spectrum is collected under high pH (pH=13) conditions. However, soil P_omay be hydrolyzed under high pH conditions, which affects the accuracy of the P-NMR measurements. Furthermore, the pH of soils usually ranges from 6 to 8, and thus it is necessary to explore the differences in the spectra of P_ocompounds under different pH conditions. Typical P compounds including D-glucose-6-phosphate disodium (D-G-6-P), 5′-adenosine monophosphate (5′AMP) and sodium dihydrogen phosphate standards were selected for this study under different pH conditions. The results show that the pH change significantly affects spectral features of the investigated P_ocompounds. For D-G-6-P, the shape and peak positions of the NMR spectra both change under varied pH conditions, but pH mainly affects the position of 5′AMP and NaH₂PO₄absorption peaks. D-G-6-P has α-and β-forms in solutions, and transforms into glucose phosphate, mannose phosphate, fructose phosphate and saccharinic acid phosphate, but it mainly exists as 3-hydroxy-2-oxopropyl phosphate and saccharinic acid after degradation, accounting for more than 50% of the total content, at high pH. For 5′-adenosine monophosphate, there are three conformations of 5′AMP in the solution. The resolved peak at high pH probably results from the hydrolysis of 5′AMP to produce orthophosphate, while for NaH₂PO₄, the existence of and at low and high pH values leads to changes in the peak shift in the spectra. Overall, the extraction of P_ousing NaOH-EDTA solution with high pH may significantly change the speciation of P_o, and thus induce the changes in their spectra. This study provides theoretical bases for the comprehensive understanding of soil P_oNMR spectra and the development of new method to characterize soil P_o speciation at pH values approaching natural soil pH ranges.

Key words: Organic phosphorus, pH, Solution phosphorus-31 NMR spectrum, Chemical shift