Application of spin counting to the solid-state 31P NMR analysis of pasture soils with varying phosphorus content

Abstract

Solid-state 31P NMR spectroscopy has the potential to identify forms of soil P without the need for extractions or pretreatment. We used both cross polarization (CP) and direct polarization or Bloch Decay (DP) solid-state 31P NMR to examine the forms of P in a set of soil samples that vary widely in their total P contents and proportions of organic and inorganic P. Using the technique of spin counting, we found that the 31P NMR observability (Pobs) of P in our soils was poor. Average Pobs was 9% by CP and 22% by DP. We attributed the poor observability to paramagnetic iron in close association with both organic and inorganic P. Using a series of selective extractions, we assigned the broad resonances of whole soil 31P NMR spectra to organic P and prominent, sharp resonances to inorganic P. Pretreatment of soils with HF, as commonly used in 13C and 15N NMR analyses, resulted in Pobs of >70% by both CP and DP. However, organic P recovery in this fraction was poor. Our findings highlight the risks of trying to quantify different P types by integrating NMR spectra without taking into the account possible differences in their NMR sensitivity. Furthermore, we believe that significant improvements in the information garnered from solid-state 31P NMR analysis of soil will come not from improving resolution—there are fundamental limitations here—but in using information contained in nonfrequency parameters, such as observability, chemical shift anisotropy, and relaxation rates.