Phosphorus in sequentially extracted fen peat soils: A K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy study

The speciation of phosphorus (P) in native and degraded peat soils is an analytical challenge, and synchrotron‐based P K‐edge X‐ray absorption near‐edge structure (XANES) is a suitable method to gain information on P species in soils and organic materials. The objective of the present study was to test if P K‐edge XANES reflected differences in P fractions in fen peat due to sequential extraction and peat degradation. We investigated each one top‐ and subsoil sample of a Fibric Histosol, which differed in the degree of humification (H8 vs. H5) and concentration of total P (P_t) (1944 mg kg^–1 vs. 436 mg kg^–1). In the topsoil, residual P, H₂SO₄‐P, and NaOH‐P accounted for roughly the same proportions of P_t (≈30%). In the subsoil, residual P (64% of P_t) was more abundant than NaOH‐P (21% of P_t) and H₂SO₄‐P (10% of P_t). Among many different P reference standards, the P XANES spectra reflected differences in mineral P more distinctive than in organic P compounds. Phosphorus XANES spectra of the residues after each sequential extraction step all showed a prominent white‐line peak at around 2152 eV. Stepwise removal of resin‐P, NaHCO₃‐P, and NaOH‐P were reflected mainly by the peak intensity but scarcely by distinct spectral features. Extraction with H₂SO₄ led to the disappearance of spectral features of Ca and Mg phosphates which is a first direct hint to these compounds in the peat. In conclusion, a combined sequential fractionation and spectroscopic (³¹P NMR, P K‐ and L‐edge XANES with linear‐combination fits) approach is proposed to overcome limitations of the present study and gain more insight into the P species in peat soils.