A critical review and evaluation of some P-research methods

Important tools for the investigation of phosphorus (P) in soil are the acid digestion in aqua regia (AR) for total P, the sequential P fractionation (SF), 31P nuclear magnetic resonance (31P-NMR) spectroscopy and P K-edge X-ray absorption near-edge structure (P K-edge XANES) spectroscopy. The objective of the present study was to critically compare the outcomes of these methods in order to improve the data evaluation and interpretation. For this purpose, we analyzed total P (Pt) and the P speciation of three groups of soil samples (extremely P-rich, P-poor acidic, P-poor to P-rich alkaline). The recovery of the extracted P between the different methods depended on the soil characteristics, resulting in recommendations for improving the P extraction. The 31P-NMR spectra were calibrated to a monoester standard which provided a better consistency of the peak position in comparison to the shifting of the orthophosphate peak to 6.0 ppm. The SF and 31P-NMR agreed in the inorganic P (Pi) and Po proportions, especially in the alkaline fraction. The correlation between Po from P K-edge XANES and diesters from 31P-NMR indicated that P K-edge XANES detected rather diesters than monoesters in soils. Correlations between SF and P K-edge XANES results confirmed the presence of specific Pi species in the sequentially extracted fractions. However, the SF is still relatively unspecific for interpreting Po species from a certain fraction. A more specific description of the Po composition in fractions from the SF requires investigation of these fractions by 31P-NMR and P K-edge XANES.