Alterations to Carbon, Nitrogen, and Phosphorous Biogeochemistry Across a Burn Severity Gradient: Implications for Varied Wildfire Impacts on River Corridor Hydro-Biogeochemistry

Authors

Allison Myers-Pigg^1.2* (allison.myers-pigg@pnnl.gov), Morgan Barnes³, Alan Roebuck Jr.¹, Samantha Grieger¹, John Bailey⁴, Kevin Bladon⁴, Vanessa Garayburu-Caruso³, Emily Graham³, Kathleen Munson¹, Joshua Torgeson³, Tim Scheibe³

Institutions

¹Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA; ²The University of Toledo, Toledo, OH; ³Pacific Northwest National Laboratory, Richland, WA; ⁴Oregon State University, Corvallis, OR

URLs

• https://www.pnnl.gov/projects/river-corridor

Abstract

In the Pacific Northwest, changes in wildfire regimes have highlighted the need to better understand how burned vegetation alters ecosystem nutrient dynamics. Char and ash runoff into waterways can change aquatic ecosystem function, and recent research has indicated these materials may be more bioavailable than previously considered. This study examined how wildfire influences carbon (C), nitrogen (N), and phosphorus (P) concentration and molecular composition in solid chars and their leachates to understand the amount and types of materials that could be transported to aquatic ecosystems post-fire. Using an open-air burn table, the research team generated vegetation-derived chars representing major ecotones in the Pacific Northwest (e.g., Douglas-fir forest, mixed conifer forest, mountain woodland, and sagebrush shrublands). Researchers manipulated the moisture and duration of heating when generating the chars and evaluated the differences in burn severity that resulted, using field-derived metrics of burn severity (e.g., unburned, low, moderate, high).

Findings showed that total C and N concentrations generally decreased, while P increased with increasing burn severity in the solid chars. The percentage of C, N, and P mobilized to the aqueous phase showed contrasting patterns with burn severity. As burn severity increased, the percentage of soluble P systematically decreased regardless of vegetation type, while shifts in soluble C and N were related to vegetation type. Increasing burn severity also changed the molecular composition of both the solids and leachates. In the solid chars as burn severity increased, C chemistry became less diverse, aromatic six-member N structures were generated, and P was composed of proportionally more crystalline inorganic species as measured via X-ray absorption near edge structure and nuclear magnetic resonance. Leachate composition demonstrated a loss of protein-like compounds and an increase in aromaticity (measured via Fourier-transform ion cyclotron resonance mass spectrometry) with increasing burn severity. Overall, findings indicate that shifts in C, N, and P concentration and chemistry post-fire are related to burn severity. Thus, post-fire C, N, and P dynamics may uniquely impact downstream riverine ecosystem responses with shifts in landscape burn severities expected due to changing fire regimes.