Autotrophic fixation of geogenic CO2 by microorganisms contributes to soil organic matter formation and alters isotope signatures in a wetland mofette

[摘要] To quantify the contribution of autotrophic microorganisms to organic matter (OM) formation in soils, we investigated natural CO₂ vents (mofettes)situated in a wetland in northwest Bohemia (Czech Republic). Mofette soils hadhigher soil organic matter (SOM) concentrations than reference soils due to restricteddecomposition under high CO₂ levels. We used radiocarbon (Δ¹⁴C)and stable carbon (δ¹³C) isotope ratios tocharacterize SOM and its sources in two mofettes and compared it withrespective reference soils, which were not influenced by geogenic CO₂.

The geogenic CO₂ emitted at these sites is free of radiocarbon andenriched in ¹³C compared to atmospheric CO₂. Together, theseisotopic signals allow us to distinguish C fixed by plants from C fixed byautotrophic microorganisms using their differences in ¹³Cdiscrimination. We can then estimate that up to 27 % of soil organicmatter in the 0–10 cm layer of these soils was derived from microbiallyassimilated CO₂.

Isotope values of bulk SOM were shifted towards more positive δ¹³Cand more negative Δ¹⁴C values in mofettes compared toreference soils, suggesting that geogenic CO₂ emitted from the soilatmosphere is incorporated into SOM. To distinguish whether geogenicCO₂ was fixed by plants or by CO₂ assimilating microorganisms, wefirst used the proportional differences in radiocarbon and δ¹³Cvalues to indicate the magnitude of discrimination of the stable isotopes inliving plants. Deviation from this relationship was taken to indicate thepresence of microbial CO₂ fixation, as microbial discrimination shoulddiffer from that of plants. ¹³CO₂-labelling experiments confirmedhigh activity of CO₂ assimilating microbes in the top 10 cm, whereδ¹³C values of SOM were shifted up to 2 ‰towards more negative values. Uptake rates of microbial CO₂ fixationranged up to 1.59 ± 0.16 μg g_dw⁻¹ d⁻¹. We inferredthat the negative δ¹³C shift was caused by the activity ofautotrophic microorganisms using the Calvin–Benson–Bassham (CBB) cycle, asindicated from quantification of cbbL/cbbM marker genes encoding for RubisCOby quantitative polymerase chain reaction (qPCR) and by acetogenic andmethanogenic microorganisms, shown present in the mofettes by previousstudies. Combined Δ¹⁴C and δ¹³C isotope massbalances indicated that microbially derived carbon accounted for 8–27 %of bulk SOM in this soil layer.

The findings imply that autotrophic microorganisms can recycle significantamounts of carbon in wetland soils and might contribute to observedradiocarbon reservoir effects influencing Δ¹⁴C signatures inpeat deposits.

[学科分类] 地球化学与岩石