Peat fires and legacy toxic metal release: An integrative biogeochemical and ecohydrological conceptual framework

Peatlands are potent landscape sinks of natural and industrial toxic metals and metalloids (TMMs) but the long-term sequestration of TMMs in peatlands is at increasing risk due to climate change enhanced peatland fires. The ability of peatlands to retain TMMs results from a host of interacting hydrological, biological, geomorphological, and chemical feedbacks, which underpin peatland functionality in general. Fire is a transformative force that often disrupts these interactions and feedbacks, leading to the potential release of TMMs to our air, land, and water. Given that wildfire burned area and severity are increasing there is a need for a conceptual understanding of these interactive processes. Prior to a fire, peatland TMM mobility is relatively low, controlled by a peatland's degree of minerotrophy, degradation status, hydrogeomorphic setting and hydroclimate. Incidentally, these peatland characteristics also control the likelihood of peat ignition, creating important feedbacks on the landscape. Following ignition, the temperature and duration of a peat fire plays a critical role in determining the potential TMM emissions to the atmosphere and the post-fire geochemical conditions. We elucidate the varied emission factors of different metals, where emission factors range from 0.2 (Co or Cd) to 300 (Al) mg of metal per kg of particulate matter emitted depending on the specific metal and likely the pre-fire peat metal concentration. Following a peat fire, the geochemical and hydrological changes become increasingly important. For example, post-fire increases in pH play the strongest chemical role in limiting TMM mobilization but concurrent increases in dissolved organic matter aromaticity complicate our understanding of these processes, leading to a critical knowledge gap. At larger spatial scales, peatland and watershed ecohydrological connectivity and peat erosion modulate the release of TMMs to aquatic systems. Yet, the evolution of the ecohydrological connectivity and peat erosion potential as the peatland vegetation and hydrology recover to pre-fire conditions over the course of several to tens of years is governed by the same controls that impact pre-fire TMM mobility. Critically, the uncertainty in evolution trajectories depends on changes in biological, hydrological, climatological, and chemical conditions, limiting our ability to accurately predict these changes under a rapidly changing climate. This extensive and interdisciplinary review guides the development of a conceptual framework and highlights future research needs to better respond to the emerging threat of legacy TMM release from peatland wildfires.