Abstract
Solution 31P nuclear magnetic resonance (NMR) spectroscopy has been widely applied to analyze the speciation of soil organic P; however, this time-consuming technique suffers from a low analytical efficiency, because of the lack of fundamental information such as the spin–lattice relaxation (T1) of 31P nucleus for model P compounds. In this study, we for the first time determined the T1 values of twelve typical soil organic P compounds using the inversion recovery method. Furthermore, we examined the effect of co-existing paramagnetic ions (e.g., Fe3+ and Mn2+) on the reduction of the T1 values of these compounds. Without the addition of paramagnetic ions, the T1 values of twelve model P compounds ranged from 0.61 s for phytic acid to 9.65 s for orthophosphate. In contrast, the presence of paramagnetic ion significantly shortened the T1 values of orthophosphate, pyrophosphate, and phytic acid to 1.29, 1.26, and 0.07 s, respectively, except that of deoxyribonucleic acid (DNA) remaining unchanged. Additionally, we evaluated the feasibility of improving the efficiency of quantitative 31P NMR analysis via addition of paramagnetic ion. Results show that, after an addition of 50 mg L−1 paramagnetic ions, 31P NMR measurement can be 3 times more efficient, attributed to the reduced T1 and the corresponding recycle delay.
Highlights
Organic phosphorus (P) accounts for 35–65% of soil P [1], the dynamics of organic P in soils plays an important role in the global biogeochemical P cycling, which is beneficial for sustainable agriculture [2,3,4]
Chemical shifts of model P compounds Solution 31P nuclear magnetic resonance (NMR) spectra of the model P compounds were shown in Fig. 1, which were all collected in alkaline condition (1 M NaOH), except that of l-α phosphatidyl choline in neutral H 2O because it would hydrolyze in alkaline solution
The phosphonate compound resonated at 20.87 ppm, whereas the organic polyphosphate generated three characteristic peaks at − 4.07, − 9.33, and − 19.67 ppm corresponding to the three phosphates in adenosine 5′ triphosphate (ATP)
Summary
Organic phosphorus (P) accounts for 35–65% of soil P [1], the dynamics of organic P in soils plays an important role in the global biogeochemical P cycling, which is beneficial for sustainable agriculture [2,3,4]. Unlike soil inorganic P mainly existing as orthophosphates that can be identified by the classic colorimetry method [5], soil organic P that has a wide range of diverse molecular structures is difficult to analyze [6]. Advanced analytical techniques are essential to analyze the speciation and composition of organic P in the heterogeneous soils at the molecular level for comprehensive information about the P biogeochemistry in soil. In the past 20 years, solution 31P nuclear magnetic resonance (NMR) spectroscopy has been developed and widely applied to speciate soil organic P after certain chemical extractions [7,8,9]. Because the isotropic NMR signals are well resolved and indicative for the structural information of specific P compounds, soil extracts are routinely analyzed by one-dimensional (1D) solution 31P NMR to quantitatively determine different organic P species [8]. Capable of extracting most the bioavailable P in soil, a single-step
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