The impacts of fires can be extensive and long-lasting, going beyond the destruction of towns and farmland to devastating water supply catchments, resulting in long-term reductions in catchment yield [1]. As global climate change continues, all aspects of the fire regime are expected to respond immediately, leading to increased future fire impacts on soil, water, and vegetation [2]. Therefore, there is an urgent need for accurate, long-term records to aid in mitigating and recovering from future fire events.
In Australia, bushfires have played a crucial role in shaping the surface landscape and biodiversity [3]. However, recent anthropogenic climate change is likely to alter the fire regime, including changes in fire frequency, severity (amount of biomass consumed), and the duration of fire seasons [4], [5]. It is crucial to understand how fire regimes will respond to future climate changes.
Furthermore, wildfires can release accumulated heavy metals into the atmosphere and adjacent streams, potentially impacting terrestrial and aquatic ecosystem health for short periods but with high concentrations [6]. Post-fire metal concentrations and export data for receiving streams are limited. The speciation of elements plays a significant role in their transport, bioaccumulation, biomagnification, and toxicity. Wildfires are expected to cause changes in element speciation; for instance, changes in soil pH due to fire can enhance or hinder the mobility of elements such as Cu, Fe, Mn, and Zn, while oxyanions (As, Cr) may be immobilized at low pH by precipitation or adsorption onto soil colloids [7]. In addition to volatile and semi-volatile compounds, fires can produce mineral by-products such as nanoparticles (NPs), considered emerging contaminants, with potential health effects [8].
Transformations of metal ions during combustion and their speciation during wildfires are poorly documented, as most studies report only total metal concentrations [9], which are less relevant for understanding the effect of the exposome or serving as proxies for past fires.
Current methods for reconstructing past fire events, such as monitoring changes in the hydrograph, charcoal analysis, and remote sensing, have limitations in distinguishing fire severity and characteristics. To extend our records, new proxies are needed. Fourier Transform Infrared (FTIR) Spectroscopy emerges as a promising proxy for characterizing past fires. It utilizes infrared light to interact with chemical bonds in a sample, providing unique bands associated with different functional groups [10]. After a fire, the thermal decomposition and transformation of organic bonds create a signature that can approximate fire intensity and temperature [11]. This technique allows insight into fire conditions, including the presence of pyrolysis. While most FTIR studies have analyzed laboratory-produced charcoal or soil organic matter, this study seeks to fill a gap by examining the effects on bulk sediment, which occurs after soil erosion/mobilization. 

The boron isotope composition of sediment/soil deposits also holds potential as a useful proxy for identifying past fires [12]. Boron is a crucial micronutrient for plants, and changes in its isotope composition have been observed in lake sediments, possibly recording past fire events [13]. The presence of boron in charcoal and ash transferred to soil particles can be utilized to determine the severity of past fire events [14].

To address metal ion speciation during wildfires, a comprehensive and multi-analytical approach is required to effectively isolate nanoparticles from environmental matrices. Our team at IPGP has developed a unique methodology that combines single particle ICP-MS (sp-ICP-MS) and single particle ICP-Tof-MS (sp-ICP-Tof-MS) alongside agglomerative hierarchical clustering analysis. This innovative approach enables us to identify and analyze the occurrence and chemical signature of nanoparticles in aquatic samples [15],[16]. By employing this method, we aim to gain insights into the fate of elements released from soil into soil solutions or rivers during fires. Furthermore, this technique holds great promise in using these elements as proxies for assessing fire intensity and identifying potential sources of toxic elements.

The aim of this project is to investigate the potential of FTIR spectroscopy and the boron isotope composition of detrital matter in fire-affected soils/sediments, in combination with sp-ICP-Tof-MS, scanning, and transmission electron microscopy observations, to identify the characteristics of past fire events. Our main objectives are as follows:
• Analyze FTIR spectra in soils/sediment layers deposited after a fire event to detect shifts from aliphatic to aromatic compounds and thermal decomposition of various bonds.
• Investigate changes in the B isotope composition of clays, as combustion processes release a significant amount of B in volatile form.
• Examine contributions from plant matter, which are expected to vary with severity and reflect exposure to elevated temperatures.
• Develop a unique approach utilizing single particle ICP-MS (sp-ICP-MS) and single particle ICP-Tof-MS (sp-ICP-Tof-MS) coupled with agglomerative hierarchical clustering analysis to better isolate nanoparticles from environmental matrices.
• Utilize nanoparticles in aquatic samples as proxies for fire intensity or the nature of the source material burned during the fires.
• To validate these techniques, we will conduct tests on samples from areas affected by a high severity fire event in Australia in 2013 and in France in 2022. By achieving our objectives, we aim to enhance our understanding of past fire events and gain valuable insights into the effects of fires on soil/sediment compositions.

In this study, our sampling strategy aims to investigate the effects of wildfires on soil and water in two different regions: Yengo National Park, Australia, and the Landes de Gascogne forest, France.

– Yengo National Park, Australia: We will collect soil clay fractions from various sites within Yengo National Park. These sites have previously experienced different combinations of low and high severity fires during the 1993/1994 and 2001/2002 fire seasons. Additionally, these sites have been studied before to understand the patterns of soil carbon flux following bushfires. For our analysis, we will focus on soil samples taken from 10 different top layers (0-5cm depth) and one core sample (50cm depth) representative of the watershed pedology. We will also gather residues of combustion that remain on the soil surface. Moreover, we will collect surface water samples from nearby rivers, as well as lack waters.
– Landes de Gascogne Forest, France: The Landes de Gascogne forest in France experienced significant wildfires in 2022, particularly impacting two major sites: “La Teste de Buch” and “Landiras.” Notably, an underground fire is still active in this region. The Landes region is characterized by its production of “Pin Maritime” and has sandy soil with shallow groundwater aquifers. To investigate the impact of the 2022 wildfires on this region, we will collect soil samples from burnt sites in “La Teste de Buch” and “Landiras.” Similar to the Australian site, we will gather 10 top soil samples (0-5cm depth) and one core sample (50cm depth) representing the watershed pedology. Additionally, we will collect residues of combustion found on the soil surface. To assess the impact on water sources, we will collect surface water samples from the La Leyre River and its tributaries. Furthermore, we will collect groundwater samples, taking into account the varying depths of the water table in these regions.
By adopting this comprehensive sampling strategy, we aim to gain valuable insights into the effects of wildfires on soil and water compositions in these two distinct regions.

Scientific: This project will advance our knowledge of wildfire severity. This project will provide original proxies to be used to understand past, present and future fire impacts in the critical zone.

Experimental data: The results will be available for use in studies or models of forest emissions and biomass combustion.

Technical: Methodological advances: to support the development of analytical techniques for monitoring or water treatment laboratories. This could lead to better risk management or even to new ways of combating the unstoppable megafires.

Societal: This work will provide elements relating to the behaviour of pollutants emitted into the environment, such as their persistence.

Mitigation solutions: These data are expected by the managers/operators of drinking water production plants in the regions affected by forest fires. 

Agricultural practices: Our results are relevant for institutions that are responsible for monitoring the integrity of the forest estate, the preservation of structures and the protection of forest stands.

Policy-making: This project responds to the theme of national and social resilience, which includes risk identification, crisis management and recovery. Thus, by supporting research on the propagation of forest fires and prospective research on new means of fire-fighting, this project aims to limit the huge impacts of forest fires, including climate change, which is exacerbated by global emissions from forest fires.

Institut de physique du globe de Paris (IPGP)

Australia

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