Aerobic and anaerobic burning alter trace metal availability in peat soils: evidence from laboratory experiments

Abstract

As global warming becomes more pronounced, climate change and human activities are leading to frequent peat fire incidents. Fire plays an important role in the environmental distribution of trace metals in peat soils. In the current study, we collected peat soils from six peatlands of the Great Khingan Mountains in Northeast China, where wildfires have often occurred in recent decades. To investigate the transformation of trace metals in peat soils by fire, burning experiments at 250°C (light) or 600°C (severe) and under aerobic (AE, flaming) and anaerobic (AN, smouldering) conditions (AE250, AE600, AN250, AN600) were carried out in the laboratory to investigate the effect of different burning intensities on the distribution of Cu, As, Pb, and Cd in peat soils. The European Community Bureau of Reference (BCR) sequential extraction method for metal fractions was applied for partitioning four fractions (exchangeable, reducible, oxidizable, and residual). The results showed that AE600 significantly decreased the percentages of oxidizable Cu, As, Cd and Pb (Cuoxi, Asoxi Cdoxi and Pboxi) compared with those of the original samples, e.g., Cuoxi decreased from 68.84% ± 12.76% to 15.82% ± 8.02% in peat under moss, which might result from organic matter decrease. Under AN250 conditions, the exchangeable As (Asexc) and Pb (Pbexc) significantly increased by more than twice. Different burning intensities significantly changed the various fractions of Cu, e.g., more than 20% Cuoxi transferred to exchangeable and reducible fractions as the temperature increased. Most of the available fractions of As transformed to residual As after burning. Our study suggests that the AE600 treatment increased the ecotoxicity and bioavailability of Cu, Pb and Cd in peat soils, while AN250 burning decreased the potential ecotoxicity of Cu, As, and Pb. Compared with lower temperature flaming burning (AE250), the ecotoxicity and bioavailability of Cu and Pb were greater in high temperature (AE600) conditions. Smouldering fires at both temperatures (AN250 and AN600) maintained a high potential ecotoxicity and bioavailability of Cu and Pb in peat soils.

Globally-significant arsenic release by wildfires in a mining-impacted boreal landscape

“…In the Taiga Shield, water in the uplands often runs off into low-lying wetlands (Spence andWoo 2003, 2006), where arsenic can accumulate or transform to more bioavailable or mobile species (As(III) and organic-arsenic) due to the reducing conditions (Mikutta and Rothwell 2016). The additional post-fire upland arsenic input, coupled with large increases in water-soluble arsenic species following wetland fire (Li et al 2023), may result in a significant source of (bio)available arsenic to surface waters after wildfires. The combination of direct arsenic-rich runoff and erosion from uplands and the runoff directly from wetlands may pose a significant threat to regional drinking water quality and have potentially disastrous cascading consequences on aquatic ecosystem services (e.g.…”

“…In the case of biomass, ρ b and D were replaced by aboveground dry biomass (B; kg m −2 ) Quantifying the proportion of arsenic that may have been released to the atmosphere or water requires knowledge of fire temperature, redox status, soil mineralogy, and arsenic speciation. Given that observations of these characteristics are inadequate or entirely absent, we present a range of low and high estimates derived from laboratory combustion experiments on peat (Li et al 2023), forested mineral soils (Johnston et al 2018(Johnston et al , 2019, and wood (Chen et al 2019). Atmospheric emission estimates from peat were supplemented by field studies of arsenic emission from peatland wildfires (Lestari et al 2024).…”

“…Not only is arsenic among the most potent toxins, as it can be metabolized inside the human body (Chen and Costa 2021), but it is also widespread in the geosphere (Matschullat 2000) leading to elevated soil and water arsenic concentrations in regions surrounding historical smelting operations (Wang and Mulligan 2006, Zhang et al 2009, Sprague and Vermaire 2018. Wildfires can result in arsenic remobilization through emissions to the atmosphere, transformation to watersoluble arsenic compounds, and can be bound to eroded sediments in both upland forest and wetland soils (Johnston et al 2018, 2019, Pennino et al 2022, Li et al 2023. The specific wildfire-induced arsenic mobilization pathway depends on arsenic speciation, redox status, wildfire temperature, and the ecosystem, thus soil type (Pennino et al 2022).…”

“…peatlands) is often present as reduced As(III) sulfide minerals, organic arsenic, or complexed with organic matter (Langner et al 2012, Mikutta andRothwell 2016). In high temperature (∼600 • C) peatland wildfires, the majority of arsenic (∼80%) can be emitted into the atmosphere, while the remaining arsenic is mostly immobilized within the peat structure as less reactive or converted to soluble arsenic species (Li et al 2023). However, most peatland fires are smouldering fires that have relatively low temperatures (Huang and Rein 2017) and produce a large amount of PM (Hu et al 2018).…”

“…However, most peatland fires are smouldering fires that have relatively low temperatures (Huang and Rein 2017) and produce a large amount of PM (Hu et al 2018). These low temperature peat fires (∼250 • C) dramatically increase the water mobilizable fractions (∼20%) and lowers the proportional emissions to the atmosphere (∼5%-20%) (See et al 2007, Betha et al 2013, Li et al 2023. Nevertheless, these differences between upland and wetland fire temperature and soil redox may lead to variations in overall arsenic mobilization within a landscape.…”