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| Titel |
Black (pyrogenic) carbon: a synthesis of current knowledge and uncertainties with special consideration of boreal regions |
| VerfasserIn |
C. M. Preston, M. W. I. Schmidt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 3, no. 4 ; Nr. 3, no. 4 (2006-09-19), S.397-420 |
| Datensatznummer |
250001155
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| Publikation (Nr.) |
 copernicus.org/bg-3-397-2006.pdf |
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| Zusammenfassung |
| The carbon (C) cycle in boreal regions is strongly
influenced by fire, which converts biomass and detrital C mainly to gaseous
forms (CO2 and smaller proportions of CO and CH4), and some
1–3% of mass to pyrogenic C (PyC). PyC is mainly produced as solid
charred residues, including visually-defined charcoal, and a black carbon
(BC) fraction chemically defined by its resistance to laboratory oxidation,
plus much lower proportions of volatile soot and polycyclic aromatic
hydrocarbons (PAHs). All PyC is characterized by fused aromatic rings, but
varying in cluster sizes, and presence of other elements (N, O) and
functional groups. The range of PyC structures is often
described as a continuum from partially charred plant materials, to
charcoal, soot and ultimately graphite which is formed by the combination of
heat and pressure. There are several reasons for current interest in
defining more precisely the role of PyC in the C cycle of boreal regions.
First, PyC is largely resistant to decomposition, and therefore contributes
to very stable C pools in soils and sediments. Second, it influences soil
processes, mainly through its sorption properties and cation exchange
capacity, and third, soot aerosols absorb solar radiation and may contribute
to global warming. However, there are large gaps in the basic information
needed to address these topics. While charcoal is commonly defined by visual
criteria, analytical methods for BC are mainly based on various measures of
oxidation resistance, or on yield of benzenepolycarboxylic acids. These
methods are still being developed, and capture different fractions of the
PyC structural continuum. There are few quantitative reports of PyC
production and stocks in boreal forests (essentially none for boreal
peatlands), and results are difficult to compare due to varying experimental
goals and methods, as well as inconsistent terminology. There are almost no
direct field measurements of BC aerosol production from boreal wildfires,
and little direct information on rates and mechanisms for PyC loss.
Structural characterization of charred biomass and forest floor from
wildfires generally indicates a low level of thermal alteration, with the
bulk of the material having H/C ratios still >0.2, and small aromatic
cluster sizes. Especially for the more oxidation-resistant BC fraction, a
variety of mainly circumstantial evidence suggests very slow decomposition,
with turnover on a millennial timescale (in the order of 5–7 ky), also
dependent on environmental conditions. However, there is also evidence that
some PyC may be lost in only tens to hundreds of years due to a combination
of lower thermal alteration and environmental protection. The potential for
long-term PyC storage in soil may also be limited by its consumption by
subsequent fires. Degraded, functionalized PyC is also incorporated into
humified soil organic matter, and is transported eventually to marine
sediments in dissolved and particulate form. Boreal production is estimated
as 7–17 Tg BC y−1 of solid residues and 2–2.5 Tg BC y−1 as
aerosols, compared to global estimates of 40–240 and 10–30 Tg BC y−1,
respectively. Primary research needs include basic field data on PyC
production and stocks in boreal forests and peatlands, suitable to support C
budget modeling, and development of standardized analytical methods and of
improved approaches to assess the chemical recalcitrance of typical chars
from boreal wildfires. To accomplish these goals effectively will require
much greater emphasis on interdisciplinary cooperation. |
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