11:00 AM - 11:15 AM
[MIS03-04] Distinctive pools and chemical species of phosphorus among density fractions of allophanic and non-allophanic Andisols
Keywords:phosphorus, NMR, chemical speciation
Allophanic and non-allophanic Andisols collected from Tsukuba and Osaki, respectively, were fractionated by sodium polytungstate into five density levels including 1.6-1.8, 1.8-2.0, 2.0-2.25, 2.25-2.5, and >2.5 g cm-3. Phosphorus in each density fraction was extracted sequentially by deionized water, 0.5 M NaHCO3, 0.1 M NaOH and 1.0 M HCl. After the extracts were filtered, the concentration of inorganic P (P_) in all fractions was determined colorimetrically with a molybdenum blue method. The concentration of total P (Pt) in each fraction was determined by the same method after the solution was treated using H2SO4-persulphate digestion. The concentration of organic P (Po) was calculated as the difference between Pt and P_ of each fraction.
The total concentration of P was similar in the allophanic and non-allophanic Andisols (6.2 g kg-1). A large proportion of P_ and Po in the bulk and each density fraction was extracted by NaOH, indicating that P in both Andisols was mainly associated with Al and Fe (oxy)hydroxides. The density fraction that accumulates P was contrastingly different between allophanic and non-allophanic Andisols where over 90% of P_ and Po in the former was accumulated in the >2.0 g cm-3 fraction, whereas about 70% of P_ and Po in the latter was found in the <2.0 g cm-3 fraction. According to the 31P-NMR analysis, ortho-P monoesters were the primary organic P species for the allophanic and non-allophanic Andisols, although it was 2-folds more abundant in the latter than the former. In the non-allophanic Andisols, myo-inositol hexakisphosphate, an ortho-P monoester, was accumulated in the 1.8-2.25 cm-3 fraction.