Wildfire impacts to soil in Alberta’s southern Rocky Mountains: pyrogenic carbon abundance and character on a post-wildfire landscape

Abstract

Pyrogenic carbon (PyC) is a ubiquitous soil constituent produced as a result of wildland and anthropogenic fire. The dynamic nature of wildfire produces heterogenous chemical by-products in a broad range of sizes and chemical makeup, from lighter soots to heavier charcoals. A large component of the global soil organic carbon pool, PyC is an important component of healthy soils, particularly in fire-adapted ecosystems where flora and soil fauna make use of its many positive impacts. The stock, character, and transport of PyC is poorly understood at landscape and local scales, particularly in mountainous regions and sub-alpine ecosystems. This study assessed watersheds of Waterton Lakes National Park affected by the Kenow Wildfire in 2017 and unburned reference watersheds in the adjacent Castle Provincial Park. Soil was sampled at two time periods, 2018 from 0-5 cm and 2021, from 0-5 and 5-15 cm in depth and analyzed for total carbon, total nitrogen, particle size distribution, and PyC quantity via chemical oxidation and chemical composition via pyrolysis-GCMS. Sites paired to sample locations chosen by Parks Canada in 2018 were assessed for slope and aspect characteristics. Relationships between time, depth, burn status, soil texture class, and total carbon were evaluated using two-sample t-tests, ANOVA, regression analysis, non-metric multidimensional scaling, and permutational analysis of variance. Analytical methods were contrasted to determine the most appropriate pyrolysis-GCMS pretreatment methodology for the determination of PyC chemical constituency by comparing results in a distance matrix. Soils collected from burned watersheds showed enrichment of pyrolysates directly following wildfire and four years afterwards, where PyC made up a larger portion of soil TC. Levels of benzene/aromatics and PAHs were observed to decrease on short timescales in soils from 0-5cm, leading to substantial increases in these compound families in soil from 5-15 cm compared to reference soils consistent with known vertical redistribution mechanisms common to more condensed PyC species. The PyC content of soil total carbon decreased across a gradient of time since fire, translocated and transformed via abiotic and biotic processes, and potentially diluted by OM deposition. No site characteristic was found to be a significant driver of PyC production movement or storage, however increasing percent PyC was loosely correlated with increasing soil coarseness. This study provides a first look at PyC content and character in this region of the Southern Rocky Mountains and valuable insight into the dynamics regarding movement and storage of this essential soil component.