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

Dark, that is, nonphototrophic, microbial CO ₂ fixation occurs in a large range of soils. However, it is still not known whether dark microbial CO ₂ 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 ₂ fixation in temperate forest soils, to determine the relationship between the soil CO ₂ concentration and dark microbial CO ₂ fixation, and to estimate the relative contribution of different microbial groups to dark CO ₂ fixation. For this purpose, we conducted a ¹³ C-CO ₂ labeling experiment. We found that the rates of dark microbial CO ₂ fixation were positively correlated with the CO ₂ concentration in all soils. Dark microbial CO ₂ 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 CO ₂ 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 ₂ was mostly fixed by gram-positive bacteria, and not by fungi. In conclusion, our study shows that the dark microbial CO ₂ fixation rate in temperate forest soils increases in periods of high CO ₂ concentrations, that dark microbial CO ₂ fixation is mostly accomplished by gram-positive bacteria, and that dark microbial CO ₂ fixation contributes to the formation of soil organic matter.
(© 2019 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)