[M-IS21_28PM2] Biogeochemistry

[MIS21-20] Determination of phosphorus species and bioavailavility in allophanic and non-allophanic Andisols

*Akira TAKAMOTO¹, Yohey HASHIMOTO¹, Rota WAGAI² (1.Tokyo University of Agriculture and Technology, 2.National Institute for Agro-Environmental Sciences)

Keywords:phosphorus, ecosystem, Andisols

Andisols have high phosphorus (P) retention capacity due to abundant active aluminums (Al) and irons (Fe). Such characteristics result in a significant inhibition of plant growth in Andisols, if not properly managed. Andisols are categorized into two groups on the basis of the difference in the clay mineral compositions. One group is called as allophanic Andisols, including allophane and imogolite in the clay fraction. The other is called as non-allophanic Andisols, including Al- and Fe- humus complexes and 2:1 phyllosillicates. These soil colloids are considered a major cause of high P retention capacity of allophanic and non-allophanic Andisols. Soil P forms have been investigated using chemical extraction methods. Chemically extracted P fractions of H₂O-P and NaHCO₃-P are considered readily soluble P, while NaOH-P is modelately labile P associated with Al and Fe, and HCl-P is apatite-like P. However, there are no studies determining chemical species and hosting phases of P in allophanic and non-allophanic Andisols at the molecular levels. This study was conducted to characterize the species and sorption hosts of P in allophanic and non-allophanic Andisols using Hedley's sequential P extraction method, solution ³¹P-NMR and X-ray absorption near-edge structure (XANES) spectroscopy. For revealing the behavior of P in soils precisely, it is required to separate the different soil colloids along with their density and then identify P speciation and hosting mineral phases. This study used a density separation method that can classify soil colloids including humus and Al/Fe (oxy)hydroxides by their density. The total concentration of P in the allophanic and non-allophanic Andisols was 6.2 g P kg^-1. The sequential fractionation of bulk soil showed that the largest P pool of both allophanic and non-allophanic Andisols was NaOH-P. The density fractions of 2.0-2.25, 2.25-2.5, and > 2.5 g cm^-3 accounted for 88% of allophanic Andisols, and among five fractions, the 2.0-2.25 g cm^-3 fraction was largest (44%). On the other hand, the density fractions of 1.8-2.0, 2.0-2.25, 2.25-2.5, and > 2.5 g cm^-3 accounted for 88% of non-allophanic Andisols. The sequential fractionation of allophanic Andisols showed that the NaOH fraction had a large proportion of inorganic P (Pi, 76-92%) and organic P (Po, 72-99%). The sequential fractionation of non-allophanic Andisols also showed NaOH-Pi (46-83%) and Po (54-97%) were consisted largely of phosphorus pool, with exceptions in >2.25 g cm^-3 fractions. The results combined with the density separations and sequential extraction indicated that i) P in allophanic and non-allophanic soils is primarily associated with Fe and Al minerals, ii) Pi and Po in the 2.0-2.25 g cm^-3 fraction accounted largely for the total P of allophanic Andisols (Pi: 61%, Po: 68%), iii) Pi and Po in 1.8-2.0 g cm^-3 fraction accounted largely for allophanic Andisols (Pi: 48%, Po: 64%). According to the solution ³¹P-NMR results, orthophosphate monoester accounted largely for Po in allophanic and non-allophanic Andisols. Further investigations on XANES and NMR spectroscopy will be presented for more detailed P speciation in the soils.