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| Titel |
Towards explaining excess CO2 production in wetlands – the roles of solid and dissolved organic matter as electron acceptors and of substrate quality |
| VerfasserIn |
Klaus-Holger Knorr, Chuanyu Gao, Svenja Agethen, Michael Sander |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140312
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| Publikation (Nr.) |
 EGU/EGU2017-3682.pdf |
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| Zusammenfassung |
| To understand carbon storage in water logged, anaerobic peatlands, factors controlling
mineralization have been studied for decades. Temperature, substrate quality, water table
position and the availability of electron acceptors for oxidation of organic carbon have been
identified as major factors. However, many studies reported an excess carbon dioxide (CO2)
production over methane (CH4) that cannot be explained by available electron acceptors, and
peat soils did not reach strictly methanogenic conditions (i.e., a stoichiometric formation ratio
of 1:1 of CO2 to CH4). It has been hypothesized that peat organic matter (OM) provides a
previously unrecognized electron acceptor for microbial respiration, elevating CO2 to CH4
ratios. Microbial reduction of dissolved OM has been shown in the mid 90’s, but only
recently mediated electrochemical techniques opened the possibility to access stocks
and changes in electron accepting capacities (EAC) of OM in dissolved and solid
form. While it was shown that the EAC of OM follows redox cycles of microbial
reduction and O2 reoxidation, changes in the EAC of OM were so far not related
quantitatively to CO2 production. We therefore tested if CO2 production in anoxic peat
incubations is balanced by the consumption of electron acceptors if EAC of OM is
included. We set up anoxic incubations with peat and monitored production of
CO2 and CH4, and changes in EAC of OM in the dissolved and solid phase over
time.
Interestingly, in all incubations, the EAC of dissolved OM was poorly related to CO2 and
CH4 production. Instead, dissolved OM was rapidly reduced at the onset of the incubations
and thereafter remained in reduced form. In contrast, the decrease in the EAC of particulate
(i.e. non-dissolved) OM was closely linked to the observed production of non-methanogenic
CO2. Thereby, the total EAC of the solid OM pool by far exceeded the EAC of the dissolved
OM pool. Over the course of eight week incubations, measured decreases in the
EAC of total NOM could explain 22-38 % of excess CO2 production in a weakly
decomposed peat, 30-67 % of excess CO2 production in a well decomposed peat, and
>100 % of excess CO2 production in a peat that had been exposed to oxygen for
> 1 year. In this latter peat, EAC by OM explained 45-57 % of CO2 production,
while reduction of sulfate available in this material readily explained the remaining
fraction.
Despite having considerable uncertainty arising from methodological challenges, the
collected data demonstrated that accounting for the EACs of solid and dissolved OM
may fully explain excess CO2 production. As we conservatively assumed a carbon
oxidation state of zero for our budget calculations, a higher oxidation state of C
in NOM as suggested by elemental analysis would result in electron equivalent
budgets between EAC decreases and CO2 formation even closer to 100 %. A higher
oxidation state of mineralized carbon seemed especially likely for weakly decomposed
peat, as this material had higher concentrations of oxygen and showed the largest
percentage of formed CO2 that could not be explained based on OM reduction. |
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