Phosphorus Speciation in the Pools of the Walker and Syers Model.

The Walker and Syers (WS) model, developed based on sequential chemical extractions (SCEs), is widely accepted for describing soil phosphorus (P) transformation during pedogenesis. However, SCEs provide operationally-defined P pools and thus the accuracy of the WS model is questionable. Here, we used P K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the P speciation in sequential extracts of the Hedley SCEs for surface soils from the San Francisco volcanic field soil chronosequence (1, 55, 750 and 3000 ka) located in northern Arizona. Our results show that the non-occluded P pool was consistent with its interpretation in the model. However, the so-called “Ca-P” pool obtained from the sum of the diluted HCl and concentrated HCl extractions was inaccurate and misleading since it may contain large amount of Fe-P. The accuracy depended on the soil minerology that is determined by the pedogenesis stages. The percentages of the organic P (P_o) pool over total P derived from both techniques were consistent, however, the P XANES could not differentiate various P_o species due to its instinct limitation. The occluded P pool consisted of varying amounts of Ca-P, P_o, Al-P and Fe-P, which changed systematically during pedogenesis. Our findings for the first time show that the SCEs may not be able to reflect P fractions accurately and the speciation of the occluded P pool transformed consistently with pedogenesis. We suggest that the widely-accepted WS model can be misleading, and need to reassess the chemical extraction-based understanding of P dynamics during ecosystem development for both wet and dry climates.