Consumption of residual pyrogenic carbon by wildfire (original) (raw)
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Towards a global assessment of pyrogenic carbon from vegetation fires
Global change biology, 2015
The production of pyrogenic carbon (PyC; a continuum of organic carbon (C) ranging from partially charred biomass and charcoal to soot) is a widely acknowledged C sink, with the latest estimates indicating that ~50% of the PyC produced by vegetation fires potentially sequester C over centuries. Nevertheless, the quantitative importance of PyC in the global C balance remains contentious and therefore PyC is rarely considered in global C cycle and climate studies. Here we examine the robustness of existing evidence and identify the main research gaps in the production, fluxes and fate of PyC from vegetation fires. Much of the previous work on PyC production has focused on selected components of total PyC generated in vegetation fires, likely leading to underestimates. We suggest that global PyC production could be in the range of 114-379 Tg C yr(-1) , i.e. ~0.2-0.6% of the annual terrestrial net primary production. According to our estimations, atmospheric emissions of soot/black C mi...
Pyrogenic organic matter production from wildfires: a missing sink in the global carbon cycle
Global Change Biology, 2015
Wildfires release substantial quantities of carbon (C) into the atmosphere but they also convert part of the burnt biomass into pyrogenic organic matter (PyOM). This is richer in C and, overall, more resistant to environmental degradation than the original biomass, and, therefore, PyOM production is an efficient mechanism for C sequestration. The magnitude of this C sink, however, remains poorly quantified, and current production estimates, which suggest that 1-5% of the C affected by fire is converted to PyOM, are based on incomplete inventories. Here, we quantify, for the first time, the complete range of PyOM components found in-situ immediately after a typical boreal forest fire. We utilized an experimental high-intensity crown fire in a jack pine forest (Pinus banksiana) and carried out a detailed preand postfire inventory and quantification of all fuel components, and the PyOM (i.e., all visually charred, blackened materials) produced in each of them. Our results show that, overall, 27.6% of the C affected by fire was retained in PyOM (4.8 AE 0.8 t C ha À1), rather than emitted to the atmosphere (12.6 AE 4.5 t C ha À1). The conversion rates varied substantially between fuel components. For down wood and bark, over half of the C affected was converted to PyOM, whereas for forest floor it was only one quarter, and less than a tenth for needles. If the overall conversion rate found here were applicable to boreal wildfire in general, it would translate into a PyOM production of~100 Tg C yr À1 by wildfire in the global boreal regions, more than five times the amount estimated previously. Our findings suggest that PyOM production from boreal wildfires, and potentially also from other fire-prone ecosystems, may have been underestimated and that its quantitative importance as a C sink warrants its inclusion in the global C budget estimates.
Frontiers in Earth Science, 2018
Positive feedbacks between wildfire emissions and climate are expected to increase in strength in the future; however, fires not only release carbon (C) from terrestrial to atmospheric pools, they also produce pyrogenic C (PyC) which contributes to longer-term C stability. Our objective was to quantify wildfire impacts on total C and PyC stocks in California mixed-conifer forest, and to investigate patterns in C and PyC stocks and changes across gradients of fire severity, using metrics derived from remote sensing and field observations. Our unique study accessed active wildfires to establish and measure plots within days before and after fire, prior to substantial erosion. We measured pre-and post-fire aboveground forest structure and woody fuels to calculate aboveground biomass, C and PyC, and collected forest floor and 0-5 cm mineral soil samples. Immediate tree mortality increased with severity, but overstory C loss was minimal and limited primarily to foliage. Fire released 85% of understory and herbaceous C (comprising <1.0% of total ecosystem C). The greatest C losses occurred from downed wood and forest floor pools (19.3 ± 5.1 Mg ha −1 and 25.9 ± 3.2 Mg ha −1 , respectively). Tree bark and downed wood contributed the greatest PyC gains (1.5 ± 0.3 Mg ha −1 and 1.9 ± 0.8 Mg ha −1 , respectively), and PyC in tree bark showed non-significant positive trends with increasing severity. Overall PyC losses of 1.9 ± 0.3 Mg ha −1 and 0.5 ± 0.1 Mg ha −1 occurred from forest floor and 0-5 cm mineral soil, with no clear patterns across severity. Fire resulted in a net ecosystem PyC gain (1.0 ± 1.0 Mg ha −1) across aboveground and belowground components of these forests, and there were no differences among severity levels. Carbon emissions represented only 21.6% of total forest C; however, extensive conversion of C from live to dead pools will contribute to large downed wood C pools susceptible to release in a subsequent fire, indicating that there may be a delayed relationship between fire severity and C emissions. This research advances understanding of forest C loss and stabilization as PyC in wildfires; however, poor relationships between C and PyC gains or losses and fire severity highlight the complexity of fire impacts on forest C.
Ecology, 2014
Pyrogenic carbon (PyC), a major by-product of wildfires in boreal forests, plays several critical roles in soil biogeochemical processes. However, PyC properties, including its potential recalcitrance, may vary depending on its formation conditions. Our study aimed to characterize the chemical and physical properties of PyC formed under variable fire severity in Eastern Canada boreal forests; these latter represent an important fraction of fire-affected circumboreal ecosystems. A total of 267 PyC samples, produced by early-season wildfires in 2005-2007, were collected 5 years after fire from the forest floors of 14 black spruce sites distributed across Quebec, to cover the range of fire severity encountered in these forests. Early-season fires occur frequently in Eastern Canada, and are predicted to increase in regional and global scenarios of future fire regimes associated with climate change. Selected PyC samples were analyzed using elemental analysis, solid-state 13 C nuclear magnetic resonance (NMR) spectroscopy, scanning electron microscopy, and surface area analysis. The NMR spectra of the PyC collected on low-fire-severity sites were dominated by peaks indicative of cellulose, while those for PyC from higher-severity sites were dominated by a broad peak assigned to aromatic carbons. Atomic H/C and O/C ratios decreased with increasing fire severity. By comparing field samples to samples produced in the laboratory under controlled formation conditions, we were able to infer that the temperature of formation in the field was low (758-2508C). In addition, for all PyC samples, the aromatic carbon : total carbon ratio was small, suggesting that PyC produced by early-season fires in these boreal forests may be susceptible to relatively rapid degradation. Taken together, our data suggest that boreal PyC may not be as recalcitrant as previously assumed, and that its influence on soil biogeochemical processes may be short lived.
Frontiers in Forests and Global Change
Fire affects the quantity and quality of soil organic matter (SOM). While combustion of the O-horizon causes direct losses of SOM, fire also transforms the remaining SOM into a spectrum of thermally altered organic matter. Pyrogenic carbon (PyC) can resist degradation and may have important effects on soil carbon cycling. The objectives of this study are to examine the mobility of PyC. Studying the effects of wildfire is challenging due to the rapid post-fire changes in the ecosystem and lack of robust controls. We overcame those limitations by examining the Chimney Tops 2 Fire which burned 4,617 ha of the Great Smoky Mountains National Park (GRSMNP), including a National Ecological Observatory Network (NEON) site, in November 2016. We examined PyC in soils from three time points from an area burned at low-severity (pre-, immediate post-, and 11 months post-fire) and two time points from areas burned at lower to higher severity (immediate post-and 11 months post-fire). At locations with pre-fire soil samples we found that PyC increased in the O-horizon (2.22 g BPCA/kg soil) after low severity fire, which resulted in higher PyC concentrations at 5-10 cm (0.73 g BPCA/kg soil and 17.79 g BPCA/kg C) and 10-20 cm (12.19 g BPCA/kg C) of depth in the mineral soil. Sites burned at higher severity had more PyC in the O horizon relative to sites burned at lower severity (10.29 g BPCA/kg soil and 29.89 g BPCA/kg C). As a result of higher concentrations of PyC in the O-horizons burned at higher severity, statistically more PyC moved from the O-horizon to the 0-10 cm horizon from immediate to 1-year post-fire (1.37 g BPCA/kg soil and 16.10 g BPCA/kg C). Lastly, the depth profile of C and BPCA suggest a shift in the source and amount of PyC in these soil profiles over time-possibly as a result of fire suppression. Results indicate that low severity fire may be an important mechanism by which PyC is produced and transported into mineral soils.
Pyrogenic carbon in wildfire ash: characteristics and potential as C sink
Forest fires release substantial amounts of carbon (C). Much of it is emitted to the atmosphere, but some is deposited within ash on the ground. Little is known about amount and types of C deposited in ash. Here, we quantify total C, and total inorganic, water-soluble and particulate organic fractions deposited in ash during the catastrophic 2009 'Black Saturday' wildfires in Australia. These fires coincided with the highest air temperatures and lowest humidity ever recorded in the local area, which, combined with high fuel loads of mostly long unburnt eucalypt forests, generated extreme burning conditions. In three mixed-species eucalypt forest sites sampled, the canopy, understorey and litter fuels were almost completely consumed, resulting in substantial ash deposition (mean, 81.9 t ha -1 ), with 5.9 t ha -1 of C being transferred from vegetation to the forest floor. In five temperate rainforest sites sampled, the canopy was not burnt and ash deposition was lower (mean, 48.3 t ha -1 ) than in the mixed-species eucalypt forest, but overall their higher C content resulted in higher C deposition (8.1 t ha -1 ). In all cases, most C contained in ash was organic and its pyrogenic nature infers increased resistance to degradation. Pyrogenic C is viewed by many as an important C sink, which could contribute to long-term C sequestration when incorporated into soils or sediments. Our results highlight the potential importance of the pyrogenic C pool in freshly deposited ash and, therefore, the necessity of a systematic and detailed analysis of ash deposition and C forms in ash to improve our understanding of C fluxes by forest fires.
Fire-derived organic carbon turnover in soils on a centennial scale
Biogeosciences Discussions, 2011
Pyrogenic carbon (PyC), the residue of an incomplete combustion of biomass, is considered as a carbon (C) sink due to its assumed stability in soil. PyC turnover time estimated using two modelling approaches, based on data from 16 published studies (n = 54) on PyC degradation, ranged from a decadal to centennial time scale, varying with initial biomass type, pyrolysis temperature, and incubation or field study. The average turnover time using a one-pool approach was 88 y, and the best estimate using a two-pool approach was 3 y for a fast-cycling pool and 870 y for a slow-cycling pool. Based on this meta-analysis, PyC cannot be assumed to persist in soils for thousands of years, and its use as a strategy for offsetting carbon emissions requires prudence and further research.
Forests, 2015
Pyrogenic carbon (PyC) is produced by the thermal decomposition of organic matter in the absence of oxygen (O). PyC affects nutrient availability, may enhance post-fire nitrogen (N) mineralization rates, and can be a significant carbon (C) pool in fire-prone ecosystems. Our objectives were to characterize PyC produced by wildfires and examine the influence that contrasting types of PyC have on C and N mineralization rates. We determined C, N, O, and hydrogen (H) concentrations and atomic ratios of charred bark (BK), charred pine cones (PC), and charred woody debris (WD) using elemental analysis. We also incubated soil amended with BK, PC, and WD at two concentrations for 60 days to measure C and N mineralization rates. PC had greater H/C and O/C ratios than BK and WD, suggesting that PC may have a lesser aromatic component than BK and WD. C and N mineralization rates decreased with increasing PyC concentrations, and control samples produced more CO2 than soils amended with PyC. Soils with PC produced greater CO2 and had lower N mineralization rates than soils with BK or WD. These results demonstrate that PyC type and concentration have potential to impact nutrient dynamics and C flux to the atmosphere in post-fire forest soils.
The charcoal carbon pool in boreal forest soils
Nature Geoscience, 2009
Forest fires release significant amounts of carbon dioxide into the atmosphere 1 , but also convert a fraction of the burning vegetation to charred black carbon. Black carbon is hard to break down, and formation of this reserve therefore creates a long-term soil carbon sink 2-7. However, although soil black carbon pools are important for global carbon budgets, the spatial variation and dynamics of these pools are poorly understood 6-9. Here we examine the charcoal content of 845 soil samples collected from a broad range of boreal forest landscapes and climates in Scandinavia. We show that there is considerable variation in the distribution and carbon content of soil charcoal between forest landscapes; the landscape-level amount of soil carbon stored in charcoal ranged from 0 to 222 g C m −2 , with an average of 77 g C m −2. The carbon concentration in the soil charcoal is significantly lower than that found in recently produced fresh charcoal, suggesting that charcoal carbon content decreases with time. Indeed, the median age of a subset of 14 C-dated soil charcoal particles was 652 years, implying a rapid turnover compared with the expected median age of approximately 5,000 years if charcoal is persistent. Assuming that our measurements are representative of boreal forests worldwide, we estimate that boreal forest soils store 1 Pg of carbon in the form of charcoal, equivalent to 1% of the total plant carbon stock in boreal forests. Large areas in northern forests are struck by fire every year 10. Fire return intervals of 50-200 years are common 11 , but shorter 12 or longer 13 return intervals can also prevail locally or regionally. Each fire event reduces ecosystem amounts of organic carbon (C) by release of CO 2 to the atmosphere. Such fire-driven losses of C from northern forests are huge. A fire event typically emits 1,000-2,000 g C m −2 , and with an estimated 5-15 million hectares burning annually, the inter-annual variation of emitted C is substantial 14-17. However, during each fire event a fraction of the burning vegetation and soil organic matter is converted to black pyrogenic carbon, which exists as a continuum from partly charred plant material, via char and charcoal to soot and graphite particles 6,18. As black pyrogenic carbon is refractory and stored in soils, it represents carbon that is removed from the faster cycling pools to an extent that makes it function as a long-term carbon sink 2-7. Recent findings do, however, indicate that soil pools of black pyrogenic carbon are less refractory than previously thought 6,8,18-21. This is reasonable because the amounts of pyrogenic carbon in boreal forest soils seem rather small compared with the amounts produced at each fire event 12,19 , indicating that it is re-burnt during subsequent fires, or lost by other means of degradation. Yet information on the amounts and dynamics of black pyrogenic carbon in boreal forest soils is sparse and incomplete 5,6,8. For example, existing data cannot determine the quantity of carbon present in recently produced fresh pyrogenic