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The role of CO2 fixing microbes in soil organic matter storage and dynamics near a natural CO2 vent in NW Bohemia (Czech Republic)

Martin Nowak1, Feix Beulig2, Joe v. Fischer3, Kirsten Küsel2, Susan Trumbore1
1 Abteilung für Biogeochemische Prozesse, Max-Planck Institut für Biogeochemie
2 Aquatic Geomicrobiology Group, Institute of Ecology, Friedrich Schiller University Jena, Dornburger Str. 159, D-07743 Jena, Germany
3 Department of Biology, Colorado State University, Fort Collins, CO 80523

P 6.11 in Groundwater, soil and surface water interactions

Soil respiration constitutes the biggest flux within the terrestrial carbon cycle, emitting 60 Gt of C to the atmosphere. However, it is know that part of the respired CO2 can be re-fixed by chemo-lithoautotrophic as well as heterotrophic microorganisms, which might enhance the role of soils as a carbon sink, at least locally. CO2 fixation can occur from topsoils down to deep aquifers and might therefore have also an impact on the cycling of carbon and other life sustaining elements like nitrogen, iron or sulphur in the subsurface.

Further, recycling of CO2 and its conversion into microbial biomass can alter both, the radiocarbon and stable isotope composition of SOM, as soil CO2 can be isotopically distinct from organic matter present in the same soil due to fractionation or mixing processes. This might in turn lead to misinterpretations of carbon dynamics and turnover rates determined by isotope studies.

In order to study this process, we started isotope investigations at mofette sites. Mofettes are CO2 vents of geogenic gases from the earth’s mantle. Because of the high impact of the ascending CO2 on soil properties, it was proposed to classify mofette soils as reductosols, which are characterized by the lack of oxygen, accumulation of Corg and the abundance of reduced gases. High pCO2 values up to 1 also highly influence the microbial community in these soils, shifting it towards groups that can utilize the CO2 for metabolic and catabolic reactions. Another special feature is that the soil CO2 at mofett sites is characterized by a distinct isotopic value than C that is derived from plant input, because the CO2 is “dead” with respect to radiocarbon (Δ14C = -1000‰) and enriched with respect to 13C (δ13C = -2 ‰).

We hypothesise that the amount of SOM derived from microbially fixed CO2 can be quantified by combined 14C and 13C analyses. Mass balances with both isotopes allow quantification of SOM derived from CO2 fixation, if all end-members are known. Further, we want to conduct 13CO2 as well as 14C labelling experiments with mofette soils. With this, we want to gain information about CO2 uptake rates and kinetics of the soil microbial community.

These combined field and experimental investigations will give us insights into carbon dynamics in reductosols and the importance of CO2 fixation for SOM formation. The knowledge about the fate of microbially fixed CO2 might also be important for anthropogenic reductosols that appear above leaking gas pipes or CO2 storage reservoirs.

last modified 2013-11-07