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

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 ( 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). 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.