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Contributed Talk

Session: : Climate Change: Biogeochemical Cycles 3

COS 210-3 - Burn severity alters phosphorus composition and decreases solubility of vegetation char leachates

Wednesday, August 9, 2023
4:00 PM – 4:15 PM PDT
Location: B116
Abstract: Wildfires alter organic and inorganic nutrients through incomplete combustion of biomass and enhanced transport of materials from terrestrial to aquatic environments during leaching and erosion events. As wildfire regimes change in the Pacific Northwest, it is critical to understand how burned vegetation alters ecosystem nutrient dynamics. There have been relatively few investigations on how phosphorus (P) biogeochemistry is altered post-fire. Field observations have identified impacts to P concentration and composition in burned landscapes, yet an understanding of how P systematically changes across different burn severities is lacking. Therefore, we examined how burn severity alters the mobility of P in soluble and particulate fractions produced from laboratory leachates of charred materials. We manipulated vegetation type, fuel moisture, and heating duration in open-air burn tables producing a range of burn severities of the chars. We measured total and molybdate-reactive P in the soluble and particulate fractions of leachates, and quantified shifts in P chemical speciation using nuclear magnetic resonance (NMR) and x-ray absorption near edge structure (XANES). Burning resulted in a 2- to 7-fold decrease in mean total P mobilized in the soluble phase but a 1.1- to 9-fold increase in particulate bound P export across all vegetation types. Molybdate-reactive P followed similar trends as total P, where the soluble phase decreased 1.8- to 6-fold while particulate bound P increased by 2.2- to 30-fold with burning. The soluble and particulate fractions were primarily composed of inorganic P species across all samples, identified by both NMR (soluble: 91 ± 6%) and XANES (soluble: 89 ± 16%; particulate: 84 ± 9%), with inorganic calcium-bound P species dominating both fractions of char leachates (soluble: 55 to 68%; particulate: 57 to 82%). The proportion of soluble organic P increased in low severity burns (14 ± 2%) relative to the unburned (1.5%); however soluble organic P decreased as burn severity increased (high severity = 4% organic). Increasing burn severity created systematic shifts in P biogeochemistry where higher proportions of inorganic calcium-bound species resulted in less export of P in the soluble phase and greater loss of particulate bound P. These results indicate a transformation to the potential export and reactivity of P in the environment post wildfire.