Analysis of Potentially Mobile Phosphorus in Arable Soils Using Solid State Nuclear Magnetic Resonance

In many intensive agroecosystems continued inputs of phosphorus (P) over many years can significantly increase soil P concentrations and the risk of P loss to surface waters. For this study we used solid‐state 31 P nuclear magnetic resonance (NMR) spectroscopy, high‐power decoupling with magic angle spinning (HPDec–MAS) NMR, and cross polarization with magic angle spinning (CP–MAS) NMR to determine the chemical nature of potentially mobile P associated with aluminum (Al) and calcium (Ca) in selected arable soils. Three soils with a range of bicarbonate‐extractable Olsen P concentrations (40–102 mg P kg −1 ) were obtained from a long‐term field experiment on continuous root crops at Rothamsted, UK, established in 1843 (sampled 1958). This soil has a threshold or change point at 59 mg Olsen P kg −1 , above which potentially mobile P (as determined by extraction with water or 0.01 M CaCl 2 ) increases much more per unit increase in Olsen P than below this point. Results showed that CaCl 2 and water preferentially extracted Al‐P and Ca‐P forms, respectively, from the soils. Comparison among the different soils also indicated that potentially mobile P above the threshold was largely present as a combination of soluble and loosely adsorbed (protonated–cross polarized) P forms largely associated with Ca, such as monetite (CaHPO 4 ) and dicalcium phosphate dihydrate (CaHPO 4 ·2H 2 O), and some Al‐associated P as wavellite. The findings of this study demonstrate that solid‐state NMR has the potential to provide accurate information on the chemical nature of soil P species and their potential mobility.