A method to analyse the isotopic composition of oxygen associated with organic phosphorus in soil and plant material

The isotopic composition of oxygen bound to phosphorus (P) in phosphate (δ¹⁸O-P) has been used to improve our understanding of P cycling in the soil–plant system. The determination of δ¹⁸O-P in the inorganic (P_i) and organic (P_o) soil P pools obtained from soil sequential extractions would further our knowledge. Although methods exist for the purification of inorganic soil P pools, there is no published method for organic P at present. Without this information, it is impossible to determine the signature of the inorganic P released by enzyme hydrolysis. In this paper, we present a protocol for the separation and purification of the organic and inorganic P extracted by NaOH-EDTA in sequential extraction. The procedure was tested on different reference compounds as well as soil and plant materials from temperate and tropical climates. We assessed the efficiency of separation between inorganic and organic P, and of the ultraviolet (UV) hydrolysis of organic P, and the effect of possible interfering compounds. Overall, the protocol enables 30–80% of organic P extracted by NaOH-EDTA to be purified. The measured δ¹⁸O-P of organic P ranged from 12.7 to 20.3‰ in soils and from 17 to 27‰ in plants. Although the method provides the signature of a fraction only of soil bulk organic P, it represents the best approach to assess the contribution of enzyme hydrolysis to the inorganic P pools, and it completes the set of purification schemes required to characterize δ¹⁸O-P in soil inorganic and organic P. Highlights: Streamlined method to determine the oxygen isotopic signature of organic P in soils and plants. This information is currently unavailable and values are simply ‘guessed’. The oxygen isotopic signature of organic P is not necessarily similar to that of inorganic phosphate. The signature of organic P should be determined because it enables understanding of P cycling in the soil–plant system.