Multiple burns affecting post-fire pollution cycling: Legacies of past charcoal production in areas affected by forest fires

Wildfires are a hazardous concern for human health and the environment, extensively studied for fire-prone regions for decades. However, in temperate Central Europe a significant gap remains in evaluation, assessment and understanding of the effects and risks on the geoenvironment, including post-fire pollutant cycling.  The Harz Mountains in Central Germany face this environmental challenge, due to climate change, which is driving expansion of fire-prone regions and an increase in the frequency and number of wildfires. Especially since 2022, the region has experienced wildfires following natural disturbances such as bark beetle infestation, windthrow, as well as a high frequency of heat and drought events. The landscape is shaped by legacies of land use during the past millennium. Mining, smelting and wood overexploitation phases significantly altered the topography and soils leaving widespread and partly hazardous environmental legacies, suggested to interact with modern environments, though the extent of this interaction remains poorly understood. We suggest that this interplay of recent wildfires and legacies, represented by former charcoal production sites, creates diverse fire impacts on soils within a single region. On the one hand, the widespread residues of charcoal kilns persist in the soils and on the other hand, modern wildfire affected soils again.

Our study investigates the influence of the landscape legacies in recently burnt areas by analyzing 16 priority PAHs (Polycyclic aromatic hydrocarbons) listed by the U.S. Environmental Protection Agency  in a 1.3 ha site in the Harz Mountains that burnt in 2022 (Jackisch et al., 2023). Samples of organic and mineral horizons were taken in former charcoal kiln and wildfire affected sites mapped by remote sensing. Additionally, control soil profiles were sampled. All samples were analyzed using GC-MS.

We examined the influence of heat on the mineral layer through changes in mineral composition with a focus on thermal transformation of Fe (oxy)hydroxides using SEM (scanning electron microscopy) and XRD (X-ray Diffraction) measurements in mineral layers affected by charcoal production, wildfires and non-affected soils, to improve the mapping of burn severity. We find a high heterogeneity in PAH quantities and composition due to the site’s high soil and micro-relief diversity, with high-molecular weight PAHs dominating in legacy samples.  This study contributes to the discussion about post-fire PAH cycling in soils of the Harz Mountains with legacies from past charcoal production.

Jackisch, R., Putzenlechner, B., & Dietze, E. (2023). UAV data of post fire dynamics, Quesenbank, Harz, 2022 (orthomosaics, topography, point clouds) (1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.7554598