The oxygen isotopic composition of phosphates in soils extracted with HCl

Objectives The P—O bond of PO₄^3– is a relatively steady structure in nature. The ratio between the heaviest and lightest oxygen isotopes (¹⁸O and ¹⁶O) in phosphate (Pi), i.e. the oxygen isotope composition of phosphate (δ¹⁸O-P) is used for tracing the biological use of P and deciphering P cycle in soils. The fraction of Pi extracted with HCl from soils is considered slowly-released and low bio-available for plant. To establish the determination method of δ¹⁸O-P in HCl-Pi will provide a reliable technical support for the estimation of soil P transformation.

Methods In this study, fluvo-aquic soil and black soil were sampled from the plots with four treatments including NK, NPK, NPK+straw, NPK+manure in the long-time fertilization trials located in Henan and Heilongjiang province, respectively. Soil with 1 mol/L of HCl-extractable Pi was isolated by precipitation first as ammonium phosphomolybdate (APM) and later as magnesium ammonium phosphate (MAP). The final purity of the Ag₃PO₄ precipitates was pyrolitically decomposed to carbon monoxide and analyzed for δ¹⁸O by elemental analyzer-stable isotope ratio mass spectrometer.

Results The results showed that P recovery rate in the process of three key purification precipitations was more than 92% with very low phosphate loss. There was no isotope fractionation during extraction and purification. The obtained silver phosphate sample had high purity based on X-ray diffraction and oxygen content determination. The δ¹⁸O-P values of HCl-Pi from fluvo-aquic soil and black soil were varied from 17.21‰ to 17.95‰. There was no significant difference among the four fertilization treatments.

Conclusions The established method for δ¹⁸O-P analysis of phosphates has shown satisfactory recovery and purity of HCl-Pi, and will not be impacted by the soil physical and chemical properties. Therefore, the methods could be used for the estimation of potential P cycle and transformation in the soils.