Chemistry of burning the forest floor during the FROSTFIRE experimental burn, interior Alaska, 1999

Abstract

[1] Wildfires represent one of the most common disturbances in boreal regions, and have the potential to reduce C, N, and Hg stocks in soils while contributing to atmospheric emissions. Organic soil layers of the forest floor were sampled before and after the FROSTFIRE experimental burn in interior Alaska, and were analyzed for bulk density, major and trace elements, and organic compounds. Concentrations of carbon, nutrients, and several major and trace elements were significantly altered by the burn. Emissions of C, N, and Hg, estimated from chemical mass balance equations using Fe, Al, and Si as stable constituents, indicated that 500 to 900 g C and up to 0 to 4 Â 10 À4 g Hg/m 2 were lost from the site. Calculations of nitrogen loss range from À4 to +6 g/m 2 but were highly variable (standard deviation 19), with some samples showing increased N concentrations post-burn potentially from canopy ash. Noncombustible major nutrients such as Ca and K also were inherited from canopy ash. Thermogravimetry indicates a loss of thermally labile C and increase of lignin-like C in char and ash relative to unburned counterparts. Overall, atmospheric impacts of boreal fires include large emissions of C, N and Hg that vary greatly as a function of severe fire weather and its access to deep organic layers rich in C, N, and Hg. In terrestrial systems, burning rearranges the vertical distribution of nutrients in fuels and soils, the proximity of nutrients and permafrost to surface biota, and the chemical composition of soil including its nutrient and organic constituents, all of which impact C cycling.

Black (pyrogenic) carbon: a synthesis of current knowledge and uncertainties with special consideration of boreal regions

“…Nitrogen losses from fire are highly variable and soil N may even increase from inputs of canopy necromass (Dryness et al, 1989;Harden et al, 2004). As discussed earlier, fire generally reduces the biological availability of organic N. However, thermal ammonification of organic N, increased microbial activity after reduction of thick forest floor, and inputs of freshly killed litter may result in increased availability of NH + 4 for 2-5 y (Choromanska and DeLuca, 2002).…”

“…Charring often results in only small changes (Czimczik et al, 2003;Knicker et al, 2005a) or a decrease (Baldock and Smernik, 2002;Harden et al, 2004;Neff et al, 2005) in the C/N ratio, and an increase in more recalcitrant organic N forms (González-Pérez et al, 2004;Castro et al, 2006). In SOM unaffected by fire, 15 N NMR indicates that amide is the predominant form of soil N, whereas an increase of pyrroletype N is found in fire-affected soils, and has been suggested as a molecular marker of fire (Knicker et al, 2005a).…”

“…The highly aromatic structures produced from fire were underrepresented by routine CP, and quantitative spectra required the Bloch decay (BD) method. Harden et al (2004) used thermogravimetry in O 2 to characterize fire effects on organic horizons as part of the FROST-FIRE experimental burn study in black spruce in Alaska. With burning, the proportion of mass loss at the first peak (280 • C) decreased and that at the second peak (400 • C) increased, and there was lower total mass loss up to 1000 • C. These changes indicate loss of labile C, mainly carbohydrates, and increase in lignin and/or black C. Similar changes in organic composition were found following wildfire in Alaska (Neff et al, 2005).…”

The impact of over 100 years of wildfires on mercury levels and accumulation rates in two lakes in southern California, USA

“…Wildfires and biomass burning are significant sources of Hg emissions to the atmosphere (Brunke et al 2001;Engle et al 2006;Friedli et al 2001Friedli et al , 2003aHarden et al 2004;Veiga et al 1994;Wiedinmyer and Friedli 2007). During a wildfire, Hg is primarily emitted from vegetation and soils in the form of Hg o .…”

Environmental Dynamics of Dissolved Black Carbon in Aquatic Ecosystems

“…The warming trend has led to serious consequences, among which one of the most significant is the increased forest fire frequency, especially in highlatitude regions of North America . As a key disturbance in boreal forest ecosystem, fire can cause significant carbon loss from the ecosystem (Harden et al, 2004), change the soil organic matter content and nutrients (Neff et al, 2005), produce soot that changes local and global climate (Kim et al, 2005) and generate black carbon (BC), a presumably biologically inert combustion complex from biomass burning and fossil fuel combustion . Globally, the BC production from boreal forests is estimated at 7 to 17 Tg C yr emitted as aerosols (Preston & Schmidt, 2006).…”