Soil and Litter Phosphorus‐31 Nuclear Magnetic Resonance Spectroscopy

Abstract

Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy is an excellent tool with which to study soil organic P, allowing quantitative, comparative analysis of P forms. However, for 31P NMR to be tative, all peaks must be completely visible, and in their correct relative proportions. There must be no line broadening, and adequate delay times must be used to avoid saturation of peaks. The objective of this study was to examine the effects of extractants on delay times and peak saturation. Two samples (a forest litter and a mineral soil sample) and three extractants (0.25 M NaOH, NaOH plus Chelex (Bio-Rad Laboratories, Hercules, CA), and NaOH plus EDTA) were used to determine the differences in the concentration of P and cations solubilized by each extractant, and to measure spin-lattice (T1) relaxation times of P peaks in each extract. For both soil and litter, NaOH-Chelex extracted the lowest concentrations of P. For the litter sample, T1 values were short for all extractants due to the high Fe concentration remaining after extraction. For the soil sample, there were noticeable differences among the extractants. The NaOH-Chelex sample had less Fe and Mn remaining in solution after extraction than the other extractants, and the longest delay times used in the study, 6.4 s, were not long enough for quantitative analysis. Delay times of 1.5 to 2 s for the NaOH and NaOH-EDTA were adequate. Line broadening was highest in the NaOH extracts, which had the highest concentration of Fe. On the basis of these results, recommendations for future analyses of soil and litter samples by solution 31P NMR spectroscopy include: careful selection of an extractant; measurement of paramagnetic ions extracted with P; use of appropriate delay times and the minimum number of scans; and measurement of T1 values whenever possible.

Rethinking early Earth phosphorus geochemistry

“…An estimated 25% of all marine dissolved organic P compounds are phosphonates (51,52), and 5% of organic soil P compounds are phosphonates (53). Many modern microorganisms use phosphonates as a P source when phosphate abundances are low, but most organisms will preferentially remove phosphates relative to phosphonates.…”

Extraction of soil organic phosphorus

“…This makes it particularly suitable for analysis of soil phosphorus composition by 31 P NMR spectroscopy [74]. Chelex and EDTA both release phosphorus from complexation with paramagnetic ions, although EDTA maintains paramagnetic ions in solution, which allows pulse delay times in solution 31 P NMR spectroscopy to be minimised [75]. However, extractants that include EDTA are unsuitable for co-analysis of carbon or nitrogen, for which sodium hydrox-ide alone or in combination with sodium fluoride may be preferable [72].…”

“…The most common phosphonate, 2-aminoethylphosphonic acid, is stable [55], but phosphonolipids degrade relatively rapidly, especially at elevated temperature [75]. Organic polyphosphates appear to be relatively stable in alkaline solution [55], but may precipitate with metals at high pH [116].…”

Characteristics of phosphorus components in the sediments of main rivers into the Bohai Sea

“…Generally, the P components were analyzed by solution 31 P NMR including phosphonates, orthophosphate, orthophosphate monoesters, orthophosphate diesters, pyrophosphate and polyphosphate, and the orthophosphate monoesters include some of components, such as phytate, Glucose-6-phosphate (Cade-Menun, 2005;CadeMenun and Preston, 1996;Turner et al, 2005). This method has been successfully applied for analyzed the soil, sediment (river, lake and marine) and manure (Vincent et al, 2013;Liu et al, 2015;McDowell et al, 2006;Cade-Menun et al, 2002;Ding et al, 2013;Zhang et al, 2016;Li et al, 2015).…”