Application of Spin Counting to the Solid-State 31P NMR Analysis of Pasture Soils with Varying Phosphorus Content

100% natural isotopic abundance, and has a nuclear spin of 1/2 that ensures relatively easy detection and Solid-state 31P NMR spectroscopy has the potential to identify spectral interpretation. Two different modes of 31P NMR forms of soil P without the need for extractions or pretreatment. We used both cross polarization (CP) and direct polarization or Bloch spectroscopy—solution and solid-state—have been apDecay (DP) solid-state 31P NMR to examine the forms of P in a set plied to P characterization in soils and related materials. of soil samples that vary widely in their total P contents and proporEach mode has inherent advantages and disadvantages. tions of organic and inorganic P. Using the technique of spin counting, Studies of soil P using NMR have mainly utilized soluwe found that the 31P NMR observability (Pobs) of P in our soils was tion NMR. Solution NMR spectra provide much better poor. Average Pobs was 9% by CP and 22% by DP. We attributed resolution than solid-state NMR spectra, enabling clearer the poor observability to paramagnetic iron in close association with differentiation of chemically similar species. Solution 31P both organic and inorganic P. Using a series of selective extractions, NMR spectroscopy has been widely used to differentiate we assigned the broad resonances of whole soil 31P NMR spectra to and quantify classes of organic P molecules found in organic P and prominent, sharp resonances to inorganic P. Pretreatsoils (see, for example, Cade-Menun and Preston, 1996; ment 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 Mahieu et al., 2000; Makarov et al., 2004; Turner and recovery in this fraction was poor. Our findings highlight the risks of Richardson, 2004). Impressive resolution is possible ustrying to quantify different P types by integrating NMR spectra withing this technique, for example, a range of chemically out taking into the account possible differences in their NMR sensisimilar inositol phosphates can be differentiated (Turner tivity. Furthermore, we believe that significant improvements in the and Richardson, 2004). However, in most studies, differinformation garnered from solid-state 31P NMR analysis of soil will entiation of only broad classes of organic P compounds, come not from improving resolution—there are fundamental limitasuch as orthophosphate monoesters and diesters, phostions here—but in using information contained in nonfrequency papholipids, and phosphonates, has been achieved (Caderameters, such as observability, chemical shift anisotropy, and relaxMenun and Preston, 1996; Mahieu et al., 2000; Makarov ation rates. et al., 2004). The obvious drawback to solution NMR is that only soluble species can be detected. These soluble species represent a highly variable proportion of total A variety of organic and inorganic forms of P soil P (e.g., 16.1– 99.6%) (Cade-Menun et al., 2000) and occur in soils. These play an important role in soil are typically a mixture of organic and inorganic species. P transformations, agricultural production, and environFurthermore, the high pH required to solubilize most of mental impact of land management systems. Numerous the organic matter may cause hydrolysis of sensitive phostechniques have been employed to study these forms phate esters (Makarov et al., 2002; Turner et al., 2003). and their cycling in soil. These techniques include wet The obvious attraction of solid-state NMR spectroschemical analysis, sequential fractionation, isotopic lacopy for the analysis of soil is that soil is (for the most beling, and solid-state and solution 31P NMR spectrospart) a solid, and a solid-state technique avoids the probcopy. Phosphorus-31 NMR spectroscopy has been suclems inherent with insoluble species and aggressive excessfully employed in the identification of a wide range tractants. Solid-state 31P NMR spectroscopy has found of organic and inorganic P compounds in solution. Howapplication mainly in the characterization of inorganic ever, there has been relatively little use of solid-state P in soil and other materials (Hinedi et al., 1989; Fros31P NMR to study soil despite the potential advantages sard et al., 1994; McDowell et al., 2002, 2003), although of being able to examine P compounds in situ without some researchers have used it to characterize organic P prior manipulation. (Newman and Condron, 1995; Condron et al., 1997). The Nuclear magnetic resonance spectroscopy is an ideal main disadvantage of solid-state 31P analysis is that the analytical technique for differentiating and quantifying spectra lack the resolution of solution spectra (Shand elements in different chemical environments. It is particet al., 1999). Furthermore, paramagnetic species, espeularly well suited for P characterization because the 31P cially Fe, can limit the quantitative potential of solidnucleus is NMR-sensitive (more so than 13C or 15N), has state NMR analysis of soils. This limitation has been studied in depth for solid-state 13C NMR analysis of School of Earth and Environmental Sciences, The Univ. of Adelaide, soils (see, for example, Kinchesh et al., 1995; Dai and South Australia, 5005. This research was funded by Dairy Australia. Johnson, 1999; Smernik and Oades, 2000a, 2000c). AlReceived 17 Jan. 2005. *Corresponding author (warwick.dougherty@ adelaide.edu.au). Abbreviations: CP, cross polarization; CSA, chemical shift anisotropy; DP, direct polarization or Bloch Decay; EC, electrical conductivity; Published in Soil Sci. Soc. Am. J. 69:2058–2070 (2005). Nutrient Management & Soil & Plant Analysis ICP–AES, inductively coupled plasma–atomic emission spectrometry; MAS, magic angle spinning; OC, organic carbon; Pobs, percentage of doi:10.2136/sssaj2005.0017 © Soil Science Society of America sample phosphorus observable by spin counting using 31P NMR; SSB, spinning side band. 677 S. Segoe Rd., Madison, WI 53711 USA 2058 Published online October 27, 2005