Oxygen Isotope Signatures of Phosphate in Wildfire Ash

Atmospheric aerosol deposition is a significant source of phosphorus (P) in many terrestrial and marine ecosystems worldwide, influencing their biogeochemistry and primary production. Particles emitted from wildfires (hereafter, ash) are the second most important source of atmospheric P after airborne dust. In this study, we aim to identify the signature of ash oxygen isotopes in phosphate. This will enable the use of this signature for the separation of ash from other atmospheric P sources. We measured P concentrations and δ¹⁸O_P in ash from natural and experimental fires and also from ash heated at different temperatures. The HCl and resin P concentrations (average ± SE) were 3.15 ± 0.35 and 1 ± 0.1 mg g^-1, respectively. The HCl and resin δ¹⁸O_P were 15.5 ± 0.4 and 14.7 ± 0.4‰ (average ± SE), respectively. Based on previous studies, we suggest possible isotope exchange reactions during the combustion process, between oxygen in phosphate and oxygen from other probable sources (i.e., the atmosphere, and CaCO₃ and CaO formed in the ash). The unique isotopic signature in the ash, ranging from 11.5 to 19.4‰ in the HCl and resin P fractions, is different from that of other atmospheric P sources such as airborne tree pollen, which has δ¹⁸O_P values between 19.2‰ and 29.6‰, and Saharan-dust samples collected in Israel, which have δ¹⁸O_P values ranging from 20.7‰ to 22.6‰. Thus, the δ¹⁸O_P can be used as a marker for identifying atmospheric P from wildfires and for estimating its importance to the global P cycle.