Significance of dark CO2 fixation in arctic soils. CryoCARB

Dark CO2 fixation in soil is a generally accepted process, but its importance for microbial metabolism and soil organic carbon (C) sequestration is unknown. To fill this gap, we performed screening of dark 13C-CO2 incorporation to soil organic matter in soil profiles of four tundra ecosystems around the northern circumpolar region. We further determined abundance of various soil carboxylases and identified their microbial producers. To get deeper insight into CO2 immobilization in microbial biomass, we conducted a manipulative 13C labelling experiment complemented by analyses of 13C incorporation to phospholipid fatty acids. The microbial capacity for heterotrophic CO2 fixation was determined by screening the abundance of carboxylases, analysing shifts in microbial community composition, and the incorporation of 13C into soil C following addition of available C sources. We demonstrate that dark CO2 fixation occurred ubiquitously in arctic tundra soils with increasing importance in deeper soil horizons. Dark CO2 fixation accounted on average for 0.4, 1.0, 1.1, and 16% of net respiration in the organic, cryoturbated organic, mineral and permafrost horizons, respectively. The anaplerotic enzymes of heterotrophic microorganisms comprised majority of identified carboxylases and potential for dark CO2 fixation is spread over a broad taxonomic range. Our results imply that CO2 fixation increasing with soil depth can explain isotopic enrichment of soil organic matter deposited in deeper soil horizons. Our measurements were further supported by outputs from modelling of long-term impact of dark CO2 fixation on soil organic matter.