Microscale heterogeneity of phosphorus species associated with secondary mineral phases in the B horizons of two boreal Podzols

• Poorly crystalline Al and Fe minerals have high sorption capacities for phosphate anions. However, there is still uncertainty about the molecular-level distribution of phosphorus (P) species between Al and Fe mineral phases, which can be relevant for bioavailability. The present study employed synchrotron X-ray microscopic techniques to distinguish between Al- and Fe-associated P phases in two Podzol B horizons (Flakaliden and Skogaby). First, P species were clustered into major groups depending on the main element association (Al-P, Fe-P, Ca-P + organic P) according to the color of P-containing spots in the tricolor μ-XRF quantitative maps that combined P, Al, and Fe fluorescence intensities. Second, the results were analyzed using quantitative linear combination fitting (LCF) of the P K-edge μ-XANES spectra. Third, in cases when the results diverged, a careful visual comparison between the diagnostic features of the XANES spectra of the samples and the reference standards was made. In 45 of 54 investigated spots, the color method and XANES-LCF produced consistent results. In 2 spots where the results were inconsistent, the diagnostic features of the XANES spectra agreed with the color method but not with XANES-LCF. In 39 of the 47 spots where the major P phase could be identified, Al-P was found to predominate. Ca-P was the major phase in 5 spots, Fe-P in 2, and organic P in 1. Most of the Al-P probably consisted of PO4 adsorbed to imogolite-type materials (ITM), as evidenced by oxalate extraction and Fourier Transform Infrared (FTIR) spectroscopy, although at Skogaby, adsorbed organic P may have played an important additional role. Overall, the P speciation showed substantial spatial heterogeneity, particularly in the upper B horizon of the Skogaby soil and in the lower B horizon of the Flakaliden soil. In conclusion, Al-P, particularly ITM-adsorbed PO4, is the main P phase in the B horizons of the studied Podzols, which may have implications for bioavailability as ITM is more easily dissolved than Fe oxides. Also, the combination of XANES-LCF, colors of the μ-XRF maps, and diagnostic features of the XANES spectra led to robust estimates of the major P phases in soil microsites.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geokemi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geochemistry (hsv//eng)

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Markvetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Soil Science (hsv//eng)

Nyckelord

B horizon

Imogolite-type materials

P spots

μ-XANES

μ-XRF mapping

Publikations- och innehållstyp

art (ämneskategori)

ref (ämneskategori)