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| Titel |
Effects of changing redox conditions on the dynamics of dissolved organic matter and CO2 in paddy soils |
| VerfasserIn |
Alexander Hanke, Zhi Hong Cao, Qin Liu, Jan Muhr, Karsten Kalbitz |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045356
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| Zusammenfassung |
| The current knowledge about dissolved organic matter (DOM) dynamics in soils and its
dependence on different C pools based mainly on observations and experiments in aerobic
environments. We have only a limited understanding about the effects of changing redox
conditions on production and composition of DOM although this fraction of soil organic
matter is important for greenhouse gas emission and carbon storage in soils. In many
ecosystems temporal and spatial changes of oxic and anoxic conditions are evident and might
even increase in future. It is assumed that changing redox conditions are the key drivers of
DOM dynamics in such ecosystems.
More detailed we tested the following hypotheses:
Anoxic conditions result in relative DOM accumulation due to less
mineralization of already produced DOM
Close relationship between DOM production and CO2 emission
14C signature of CO2 enables the identification of different C pools degraded at
oxic and anoxic conditions
We chose paddy soils as a model ecosystem because these soils are anoxic during the rice
growing period and oxic during harvest and growth of other crops. Furthermore, paddy soils
have oxic and anoxic horizons. Soils of a unique chronosequence of paddy soil evolution
(50 to 2000 years, China) were studied in direct comparison to non-paddy soils of
the same age. In these soils, exposed to different redox conditions over defined
periods of times, the dynamics of DOM, CO2, 14C of the CO2 and other redox
sensitive elements were followed in laboratory experiments. In the latter redox
conditions were changed every 3 weeks from oxic to anoxic and vice versa. Besides
analysis of the composition of the soil solution and the gas phase we determined
differences in C pools being respired at oxic and anoxic conditions by 14C AMS of the
CO2.
The measured redox potentials of -50 mV to 250mV at anoxic conditions and 350 mV to
550 mV at oxic conditions were in the expected range and proofed the appropriate setting of
the chosen incubation method. PH values varied between 5.5 and 7.5, where anoxic samples
had higher values than oxic ones. We further observed only small DOC contents of less than
1mg per g C. Under anoxic conditions as well as among the non-paddy soils DOC
production was slightly higher than their respective counterparts. However, we
could not find large effects of the time of rice cultivation. Nevertheless, the 2000
year old paddy soil showed highest DOC and CO2 production. The increase of
DOC and CO2 production was strongest when the oxic period disrupted the anoxic
conditions.
14C data revealed that CO2 respired from the 700 year old paddy soil was much
older than from the 2000 year old paddy soil independently from redox condition.
Furthermore, C mineralized at anoxic conditions was older than at oxic ones. During
the incubation experiment the C consumption shifted from older pools to younger
ones.
We conclude that DOM accumulated at anoxic conditions will be quickly mineralized at
oxic conditions. The influence of soil development on the C dynamics was less important
than expected, thus fresh organic matter seems to play a more decisive role. The unexpected
large decomposition of old organic matter at anoxic conditions hints to changes in the
microbial community involved. |
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