Dark CO2 fixation in soils: A meta-analysis of mechanisms and controlling factors

Dark CO2 fixation (DCF) in soil is a carbon sink process, in which microorganisms reduce CO2 to organic matter. This process occurs most at the oxygen-anoxia interface, where microorganisms oxidize reduced inorganic substrates to obtain metabolic energy. By synthesizing 215 observations from 27 peer-reviewed studies and one controlled condition study using based on 13C-, 14C-based approaches, we conducted a meta-analysis to quantify DCF rates and identify their controlling factors across continental soils. Soils exhibited an average DCF rate of 0.26 ± 0.02 μg C g⁻¹ soil d⁻¹, with the highest values observed in wetlands (0.48 μg C g⁻¹ soil d⁻¹). Across all observations, microbial biomass carbon emerged as the dominate driver of DCF, while soil depth, pH, and electron donor also contributed to its variation. DCF rates increased under higher microbial mass and moderate alkalinity but declined with depth, reflecting the influence of both metabolic activity and substrate accessibility. Hydrological regimes modulated DCF, with wetter ecosystems and stronger redox oscillations stimulating chemoautotrophic processes. Land-use and management introduced additional variation. The optimal pH for DCF differed by land-use type, peaking at 6.9 in cropland soils and 4.8 in natural ecosystems. Agricultural management shaped DCF dynamics: tillage and mineral fertilization enhanced DCF while organic amendments suppressed its activity. These patterns likely arise from distinct microbial CO2 fixation pathways, where Calvin-Benson-Bassham cycle dominates under oxic and moderately alkaline conditions, and the reductive tricarboxylic acid and Wood-Ljungdahl pathways prevail anoxic environments. Overall, DCF represents an underappreciated but ecologically relevant microbial process contributing to soil carbon formation. Incorporating DCF mechanisms into terrestrial carbon models could improve the representation of microbial carbon inputs and their feedbacks to soil carbon dynamics.