Distribution and chemical species of phosphorus across density fractions in Andisols of contrasting mineralogy

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Strong control of iron (Fe) and aluminum (Al) phases on soil phosphorus (P) dynamics is well established. How organic and inorganic P forms are associated with Fe and Al phases remains poorly understood, and sequential density fractionation thus allows one to assess the soil continuum from organic-rich particles (< 1.8 g cm−3) to organo-mineral particles (1.8–2.25 g cm−3), and finally to mineral rich particles (> 2.25 g cm−3). We combined the density fractionation approach with wet extraction techniques, 31P NMR spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy to investigate the distribution and chemical species of P in relation to Fe and Al phases for two Andisols: an allophanic Andisol rich in short-range-order aluminosilicates (silandic) and a non-allophanic Andisol rich in organic matter (OM) and organo-aluminum complexes (aluandic). We fractionated the soils to the density classes of 1.6–1.8, 1.8–2.0, 2.0–2.25, 2.25–2.5, and > 2.5 g cm−3 using the sodium polytungstate solutions. Across five density fractions, inorganic and total P were distributed in low to meso-density fractions (1.6–2.25 g cm−3) where major portions of extractable Fe and Al were found. The major P pool was found in the NaOH extraction step for low- to meso-density ranges (< 2.25 g cm−3), suggesting that Al and Fe (oxyhydr)oxides and aluminosilicates have significant roles in P sorption in allophanic and non-allophanic Andisols. In contrast, the residual P remaining after the sequential extraction was predominant in heavier-density fractions (> 2.25 g cm−3). Molybdate-reactive and total P extracted by NaOH in the density fractions were positively correlated with oxalate extractable Al (Alox) in the allophanic Andisol (r > 0.91) and with pyrophosphate extractable Al (Alp) in the non-allophanic Andisol (r > 0.97), suggesting stronger control of Al phases to P over Fe phases. The non-destructive XANES analysis confirmed that P in both Andisols was associated primarily with Al, not Fe, and suggested the presence of P bound to organo-Al complexes in the non-allophanic Andisol (up to 76% of P in the 1.6–1.8 g cm−3 fraction where Alp is enriched). Solution 31P NMR spectra of NaOH-EDTA soil extracts showed that the primary organic P group was phosphomonoesters that decreased with increasing density in allophanic and non-allophanic Andisols. Myo-inositol hexakisphosphate (IHP) and scyllo-IHP were identified in a density fraction between 1.8 and 2.25 g cm−3 of non-allophanic Andisol. Our study highlights the importance of low- to meso-density fractions in organic and inorganic P reservoirs in Andisols.