Burn severity and vegetation type control phosphorus concentration, molecular composition, and mobilization

Abstract. Shifting phosphorus (P) dynamics after wildfires can have cascading impacts from terrestrial to aquatic environments. However, it is unclear if post-fire responses are primarily driven by changes to the molecular composition of the charred material or from the transport of P-containing compounds. We used laboratory leaching experiments of Douglas-fir forest and sagebrush shrubland chars to examine how the potential mobility of P compounds is influenced by different burn severities. Burning produced a 6.9- and 29- fold increase in particulate P mobilization, but a 3.8- and 30.5- fold decrease in aqueous P released for Douglas-fir forest and sagebrush shrubland, respectively. P compound mobilization in the particulate phase was controlled by solid char total P concentrations while the aqueous phase was driven by solubility changes of molecular species. Nuclear magnetic resonance and X-ray absorption near edge structure on the solid chars indicated that organic orthophosphate monoester and diester species were thermally mineralized to inorganic P moieties with burning in both vegetation types. This coincided with the production of calcium- and magnesium-bound inorganic P compounds. With increasing burn severity there were systematic shifts in P concentration and composition— higher severity chars mobilized P compounds in the particulate phase, although the magnitude of change was vegetation specific. Our results indicate a post-fire transformation to both the composition of the solid charred material and to how P compounds are mobilized, which may influence its environmental cycling and fate.