Identification of inositol hexakisphosphate binding sites in soils by selective extraction and solution 31P NMR spectroscopy

Abstract

Inositol hexakisphosphate (IP6) can constitute the majority of the organic phosphorus in soil. Soil IP6 accumulates through a number of mechanisms, including sorption to metal hydroxides and clays, association with organic matter, and precipitation with cations and on surfaces of metal oxides. However, the relative contributions of these processes remain unknown. We quantified IP6 stereoisomers by NaOH–EDTA extraction and solution 31P NMR spectroscopy in a series of contrasting soils from natural and agricultural ecosystems, and then used selective extractions to identify associations between IP6 and soil components. Oxalic acid, which extracts amorphous and organically complexed iron and aluminum oxides, extracted the majority of the IP6 from temperate grassland and forest soils, but not from strongly weathered tropical rice soils. In contrast, removal of mineral material by pretreatment with hydrofluoric acid completely removed IP6 from temperate forest soils, but not from temperate grasslands or tropical rice soils. We conclude that the relative importance of IP6 stabilization on organic and mineral components varies markedly among soils, and that oxalate extraction provides a selective procedure for the quantification of IP6 associated with amorphous metal oxides and clays.