Identification of Phosphorus Species in Extracts of Soils with Contrasting Management Histories

Abstract

A study was conducted to characterize the relative quantity of phosphorus (P) forms and potential P loss from a series of grassland soils with a range of Olsen P concentrations (14 to 120 mg kg⁻¹), liming and fertilization histories (e.g., pH 4.1 to 6.6). Soils were extracted with 0.1 M NaOH (measure of P quantity, Q) and 0.01 M CaCl₂ (measure of P intensity, I and potentially mobile P) and analyzed by ³¹P NMR. Together, aQ/I relationship can be used to indicate when P becomes excessively mobile. Extraction efficiencies ranged from 0.9 to 1.7% of total P for 0.01 M CaCl₂ and 55 to 87% of total P for 0.1 M NaOH. A much wider range, and generally greater proportion of P as organic P, was extracted by 0.01 M CaCl₂ (phosphonate, orthophosphate, phosphate monoesters and diesters, aromatic phosphate diesters, pyrophosphate and polyphosphate) compared to 0.1 M NaOH (no aromatic phosphate diesters extracted). Forms and concentrations of P species such as the distribution of monoester- and diester-P were a function of soil management (e.g., greater concentrations of these species were noted at lower soil pH). The enhanced solubility in 0.01 M CaCl₂ relative to 0.1 M NaOH and wider range of organic P compounds of medium to high bioavailability and movement potential, shows that these species must also be considered in addition to orthophosphate when estimating a soil's potential for P loss and water quality impairment.

Acknowledgments

This work was supported in part by grants from the Foundation for Research, Science and Technology, (New Zealand, contract, AGRX0002), AGMART (New Zealand), St. John's College (Cambridge University, Cambridge, UK) and grant AO 6462 from the UK Biotechnology and Biological Sciences Research Council. The NMR instrumentation was provided by the University of London Intercollegiate Research scheme, and is located at University College (Bruker MSL 300, Dr. A.E. Aliev). The author wishes to thank Dr. N. Mahieu (Queen Mary and Westfield College, University of London, UK) for analyzing the samples by ³¹P NMR, and Dr. P.C. Brookes and Mr. P.R. Poulton (IACR-Rothamsted, UK) for allowing access to the IACR-Rothamsted, Park Grass soils.