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 contributesto 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 temperateforest soils, to determine the relationship between the soil CO2 concentration anddark microbial CO2 fixation, and to estimate the relative contribution of differentmicrobial groups to dark CO2 fixation. For this purpose, we conducted a 13C-CO2 labelingexperiment. We found that the rates of dark microbial CO2 fixation were positivelycorrelated with the CO2 concentration in all soils. Dark microbial CO2 fixationamounted to up to 320 μg C kg−1 soil day−1 in the Ah horizon. The fixation rates were2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates ofdark microbial fixation were small compared to the respiration rate (1.2%–3.9% of therespiration rate), our findings suggest that organic matter formed by microorganismsfrom CO2 contributes to the soil organic matter pool, especially given that microbialdetritus is more stable in soil than plant detritus. Phospholipid fatty acid analysesindicated that CO2 was mostly fixed by gram-positive bacteria, and not by fungi. Inconclusion, our study shows that the dark microbial CO2 fixation rate in temperateforest soils increases in periods of high CO2 concentrations, that dark microbial CO2fixation is mostly accomplished by gram-positive bacteria, and that dark microbialCO2 fixation contributes to the formation of soil organic matter.ReferenceSpohn 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