The dark side of primary production in temperate forest soils

Soils are the largest terrestrial organic carbon pool and source of atmospheric CO2 . The fate of organic carbon in soils is chiefly controlled by microbial activities, mainly via mineralization by heterotrophs releasing CO2 . However, before it is released into the atmosphere, a portion of this CO2 is refixed non-phototrophically by chemolithoautotrophic microbes. This process is called dark CO2 fixation, and is expected to not only modulate CO2 fluxes, but also contribute to soil organic carbon (SOC). Thus, CO2 fixation could be linked to the increase of SOC stocks in soils like those of temperate forests, in addition to increased plant inputs. As SOC from plant litter decreases with depth, there is a reduced availability of nutrients for heterotrophs. But since CO2 concentration increases with depth, the relative impact of chemolithoautotrophy likely increases with depth. This is expected to affect SOC and soil CO2 fluxes and might vary with changes in biogeochemical and climatic factors. Thus, quantifying the inputs and chemolithoautotrophic potential for CO2 fixation across temperate forest soil depths and the factors affecting this process, is required to determine its contribution to SOC and soil CO2 fluxes, and its response to shifts in biogeochemical parameters and global climate change. The findings presented in this thesis highlight the contributions of dark CO2 fixation to SOC by recycling CO2 produced from soil respiration across temperate forest soil profiles. Dark CO2 fixation is likely a ubiquitous process among temperate forest soil microbiome and not restricted to chemolithoautotrophs. It is influenced by changes in geochemical parameters like SOC and pH. As CO2 fixation also responds to higher CO2 concentration and temperature, this thesis further highlights that dark CO2 fixation might be affected by global climate change.