PNNL

Abstract

Shifting phosphorus (P) dynamics after wildfires can have cascading impacts from terrestrial to aquatic environments. However, it is unclear if molecular composition of the charred material or P transport is primarily responsible for observed post-fire responses. We used laboratory leaching experiments of Douglas-fir forest and sagebrush shrubland chars to examine how different burn severities influence the potential mobility of P. 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 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 both monoester and diester organic P species were mineralized with burning of both vegetation types. This coincided with the production of calcium- and magnesium-bound inorganic P species. With increasing burn severity, there were systematic shifts in P concentration and composition— higher severity chars mobilized P in the particulate phase, although the magnitude of change was vegetation specific. Our results indicate a post-fire transformation to both the mobility and bioavailability of P, relevant to its environmental cycling and fate.