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

<p>Dark, that is, nonphototrophic, microbial CO<sub>2</sub> fixation occurs in a large range of soils.<br>However, it is still not known whether dark microbial CO<sub>2</sub> fixation substantially contributes<br>to the C balance of soils and what factors control this process. Therefore,<br>the objective of this study was to quantitate dark microbial CO<sub>2</sub> fixation in temperate<br>forest soils, to determine the relationship between the soil CO<sub>2</sub> concentration and<br>dark microbial CO<sub>2</sub> fixation, and to estimate the relative contribution of different<br>microbial groups to dark CO<sub>2</sub> fixation. For this purpose, we conducted a <sup>13</sup>C-CO<sub>2</sub> labeling<br>experiment. We found that the rates of dark microbial CO<sub>2</sub> fixation were positively<br>correlated with the CO<sub>2</sub> concentration in all soils. Dark microbial CO<sub>2</sub> fixation<br>amounted to up to 320 μg C kg<sup>−1</sup> soil day<sup>−1</sup> in the Ah horizon. The fixation rates were<br>2.8–8.9 times higher in the Ah horizon than in the Bw1 horizon. Although the rates of<br>dark microbial fixation were small compared to the respiration rate (1.2%–3.9% of the<br>respiration rate), our findings suggest that organic matter formed by microorganisms<br>from CO<sub>2</sub> contributes to the soil organic matter pool, especially given that microbial<br>detritus is more stable in soil than plant detritus. Phospholipid fatty acid analyses<br>indicated that CO<sub>2</sub> was mostly fixed by gram-positive bacteria, and not by fungi. In<br>conclusion, our study shows that the dark microbial CO<sub>2</sub> fixation rate in temperate<br>forest soils increases in periods of high CO<sub>2</sub> concentrations, that dark microbial CO<sub>2</sub><br>fixation is mostly accomplished by gram-positive bacteria, and that dark microbial<br>CO<sub>2</sub> fixation contributes to the formation of soil organic matter.</p><p>Reference</p><p>Spohn M, Müller K, Höschen C, Mueller CW, Marhan S. Dark microbial CO<sub>2</sub> fixation in temperate forest soils increases with CO<sub>2</sub> concentration.<br>Glob Change Biol. 2019;00:1–10. https ://doi.org/10.1111/gcb.14937</p>