On the use of hydrofluoric acid pretreatment of soils for phosphorus-31 nuclear magnetic resonance analyses

Solid-state P-31 nuclear magnetic resonance (NMR) has the potential to be used for investigating soil P state without the need for extractions or pretreatment. Solid-state spectra typically suffer from poor resolution and low observability, however, in part due to interferences by paramagnetic species present in soil. Hydrofluoric acid treatment is routinely used in C-13 and N-15 NMR analysis of soil organic mafter to remove these paramagnetic species and improve NMR spectra. We evaluated the use of HF pretreatment to improve P-31 NMR spectra of four pasture soils. Hydrofluoric acid treatment of soils for 24 h resulted in almost complete removal of inorganic P (> 90%), but also resulted in the loss of organic P (up to 49%). Mass-balance calculations revealed that much of the organic P removed was hydrolyzed. In contrast, most model organic P compounds were found to be resistant to acid-mediated hydrolysis. Little (< 4%) of the P contained in phytic acid, beta-D-glucose phosphate, or DNA was hydrolyzed by HF in 24 h, although 29% of P in adenosine triphosphate was hydrolyzed. The observability of P (P-obs) by solid-state P-31 NMR increased on HF treatment, but was still generally poor. Attempts to analyze the HF extracts by solution P-31 NMR were unsuccessful due to the presence of high concentrations of paramagnetics. Comparison of solution P-31 NMR spectra of NaOH-EDTA extracts of the soils before and after HF treatment indicated that specific organic P compounds, in particular inositol phosphates, were removed by HE In this regard, HF treatment may have a role in the separation or fractionation of different organic P forms.