Dark microbial CO 2 fixation in temperate forest soils increases with CO 2 concentration

Dark, that is, nonphototrophic, microbial CO fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO 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 CO fixation in temperate forest soils, to determine the relationship between the soil CO concentration and dark microbial CO fixation, and to estimate the relative contribution of different microbial groups to dark CO fixation. For this purpose, we conducted a C-CO labeling experiment. We found that the rates of dark microbial CO fixation were positively correlated with the CO concentration in all soils. Dark microbial CO fixation amounted to up to 320 µg C kg soil day 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 CO 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 CO was mostly fixed by gram-positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO fixation rate in temperate forest soils increases in periods of high CO concentrations, that dark microbial CO fixation is mostly accomplished by gram-positive bacteria, and that dark microbial CO fixation contributes to the formation of soil organic matter.