Dark microbial CO2 fixation in temperate forest soils increases with CO2 concentration

Dark, that is, nonphototrophic, microbial CO₂ fixation occurs in a large range of soils.
However, it is still not known whether dark microbial CO₂ fixation substantially contributes
to the C balance of soils and what factors control this process. Therefore,
the objective of this study was to quantitate dark microbial CO₂ fixation in temperate
forest soils, to determine the relationship between the soil CO₂ concentration and
dark microbial CO₂ fixation, and to estimate the relative contribution of different
microbial groups to dark CO₂ fixation. For this purpose, we conducted a ¹³C-CO₂ labeling
experiment. We found that the rates of dark microbial CO₂ fixation were positively
correlated with the CO₂ concentration in all soils. Dark microbial CO₂ fixation
amounted to up to 320 μg C kg⁻¹ soil day⁻¹ in the Ah horizon. The fixation rates were
2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of
dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the
respiration rate), our findings suggest that organic matter formed by microorganisms
from CO₂ contributes to the soil organic matter pool, especially given that microbial
detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses
indicated that CO₂ was mostly fixed by gram-positive bacteria, and not by fungi. In
conclusion, our study shows that the dark microbial CO₂ fixation rate in temperate
forest soils increases in periods of high CO₂ concentrations, that dark microbial CO₂
fixation is mostly accomplished by gram-positive bacteria, and that dark microbial
CO₂ fixation contributes to the formation of soil organic matter.

Reference

Spohn M, Müller K, Höschen C, Mueller CW, Marhan S. Dark microbial CO₂ fixation in temperate forest soils increases with CO₂ concentration.
Glob Change Biol. 2019;00:1–10. https ://doi.org/10.1111/gcb.14937