Non-phototrophic CO2 fixation by soil microorganisms

Although soils are generally known to be a net source Of CO2 due to microbial respiration, CO2 fixation may also be an important process. The non-phototrophic fixation Of CO2 was investigated in a tracer experiment with (CO2)-C-14 in order to obtain information about the extent and the mechanisms of this process. Soils were incubated for up to 91 days in the dark. In three independent incubation experiments, a significant transfer of radioactivity from (CO2)-C-14 to soil organic matter was observed. The process was related to microbial activity and could be enhanced by the addition of readily available substrates such as acetate. CO2 fixation exhibited biphasic kinetics and was linearly related to respiration during the first phase of incubation (about 20-40 days). The fixation amounted to 3-5% of the net respiration. After this phase, the CO2 fixation decreased to 1-2% of the respiration. The amount of carbon fixed by an agricultural soil corresponded to 0.05% of the organic carbon present in the soil at the beginning of the experiment, and virtually all of the fixed CO2 was converted to organic compounds. Many autotrophic and heterotrophic biochemical processes result in the fixation Of CO2. However, the enhancement of the fixation by addition of readily available substrates and the linear correlation with respiration suggested that the process is mainly driven by aerobic heterotrophic microorganisms. We conclude that heterotrophic CO2 fixation represents a significant factor of microbial activity in soils.