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

Solid-state P-31 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 P-31 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 P-31 NMR observability (P-obs) of P in our soils was poor. Average P-obs 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 P-31 NMR spectra to organic P and prominent, sharp resonances to inorganic P. Pretreatment of soils with HF, as commonly used in C-13 and N-15 NMR analyses, resulted in P-obs 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 P-31 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.