Method for the analysis of oxygen isotopic composition of soil phosphate fractions

The isotopic signature of oxygen in phosphate (δ(18)O(P)) of various soil fractions may shed light on P transformations, including phosphorus (P) recycling by soil microorganisms, uptake by plants and P adsorption, precipitation and release by oxides and minerals, thus increasing our understanding on P cycling and lability in soils. We developed and tested a protocol to extract and purify inorganic phosphate (Pi) from different soil fractions distinguished by binding strength and precipitate it as silver phosphate (Ag(3)PO(4)) for δ(18)O(P) analysis. Soil P is extracted sequentially using water, NaHCO(3), NaOH and HCl and Pi in each solution is purified and precipitated as Ag(3)PO(4). The unique characteristics and possible interferences of the soil solution extracts are addressed. Two agricultural soil samples receiving reclaimed wastewater or fresh water were analyzed, and results indicate that all soil fractions analyzed have been impacted to some degree by biologically enzyme mediated cycling of P in the soil.