Combined applications of chemical fractionation, solution 31P-NMR and P K-edge XANES to determine phosphorus speciation in soils formed on serpentine landscapes

Abstract

Few studies have been conducted to understand phosphorus (P) dynamics in serpentinitic landscapes where soil P availability is often limited due to immobilization by abundant Fe minerals. The objective of this study was to determine soil-solid P speciation and hosting phases in the soil formed on serpentinitic landscapes using P K-edge X-ray absorption near-edge structure (XANES) spectroscopy and solution 31P nuclear magnetic resonance (NMR) spectroscopy in combination with a chemical fractionation technique. Soils formed on a mafic lithology in a temperate forest range of Mt. Asama, Mie, Japan were used. The largest P pool was found in the NaOH extractable fractions, accounting for 52–76% of total P in the soils. Both H2O and HCl fractions were a minor P pool with the average of 3.3% and 10% of total P in the soils, respectively. The NaOH–EDTA extraction for 31P-NMR recovered 31–59% of the total soil P. Orthophosphate monoesters were the major P group in soils enriched with humus, constituting 55–58% P. Inorganic orthophosphate accounted for 63% of P in the soil with abundant Fe minerals. Pyrophosphate was a minor P species accounting for 0–12% of extracted P. P K-edge XANES spectroscopy determined P adsorbed on ferrihydrite as one of the major P species in the soils. The XANES confirmed the low HCl–P concentrations determined in the chemical fractionation by unequivocal absence of apatite-like P phases in the serpentinite-derived soil. Based on the P speciation determined by the combined applications of these techniques indicate that soil P availability is severely low, and P subcycles in this serpentinitic landscape rely on orthophosphate monoesters and P associated with Fe minerals.