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

Abstract. 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 had higher soil organic matter (SOM) concentrations than reference soils due to restricted decomposition under high CO₂ levels. We used radiocarbon (Δ¹⁴C) and stable carbon (δ¹³C) isotope ratios to characterize SOM and its sources in two mofettes and compared it with respective reference soils, which were not influenced by geogenic CO₂.

The geogenic CO₂ emitted at these sites is free of radiocarbon and enriched in ¹³C compared to atmospheric CO₂. Together, these isotopic signals allow us to distinguish C fixed by plants from C fixed by autotrophic microorganisms using their differences in ¹³C discrimination. We can then estimate that up to 27 % of soil organic matter in the 0–10 cm layer of these soils was derived from microbially assimilated CO₂.

Isotope values of bulk SOM were shifted towards more positive δ¹³C and more negative Δ¹⁴C values in mofettes compared to reference soils, suggesting that geogenic CO₂ emitted from the soil atmosphere is incorporated into SOM. To distinguish whether geogenic CO₂ was fixed by plants or by CO₂ assimilating microorganisms, we first used the proportional differences in radiocarbon and δ¹³C values to indicate the magnitude of discrimination of the stable isotopes in living plants. Deviation from this relationship was taken to indicate the presence of microbial CO₂ fixation, as microbial discrimination should differ from that of plants. ¹³CO₂-labelling experiments confirmed high activity of CO₂ assimilating microbes in the top 10 cm, where δ¹³C values of SOM were shifted up to 2 ‰ towards more negative values. Uptake rates of microbial CO₂ fixation ranged up to 1.59 ± 0.16 μg g_dw⁻¹ d⁻¹. We inferred that the negative δ¹³C shift was caused by the activity of autotrophic microorganisms using the Calvin–Benson–Bassham (CBB) cycle, as indicated from quantification of cbbL/cbbM marker genes encoding for RubisCO by quantitative polymerase chain reaction (qPCR) and by acetogenic and methanogenic microorganisms, shown present in the mofettes by previous studies. Combined Δ¹⁴C and δ¹³C isotope mass balances indicated that microbially derived carbon accounted for 8–27 % of bulk SOM in this soil layer.

The findings imply that autotrophic microorganisms can recycle significant amounts of carbon in wetland soils and might contribute to observed radiocarbon reservoir effects influencing Δ¹⁴C signatures in peat deposits.