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 ¹³C-, ¹⁴C-based approaches, we conducted a meta-analysis to quantify DCF rates and identify their controlling factors across soils. Soils have an average DCF rate of 0.26 ± 0.02 μg C g^-1 soil day^-1, with the highest rates in wetlands (0.48 μg C g^-1 soil day^-1). Across all observations, microbial biomass carbon emerged as the dominant factor of DCF, while soil depth, pH, and electron donors also contributed to its variation. DCF rates increased under higher microbial biomass and moderate alkalinity but declined with depth, reflecting the influence of microbial biomass and metabolic activity as well as substrate accessibility. Hydrological regimes modulated DCF, with wetter ecosystems and stronger redox oscillations stimulating chemoautotrophic processes. Land use and management are additional factors affecting DCF intensity. 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 raised DCF rate, while organic amendments suppressed its activity. These patterns likely arise from distinct microbial CO₂ fixation pathways, where the Calvin-Benson-Bassham cycle dominates under oxic and moderately alkaline conditions, and the reductive tricarboxylic acid and Wood-Ljungdahl pathways prevail in anoxic environments. Overall, DCF represents an underappreciated but ecologically relevant microbial process contributing to organic carbon accrual in soil. Incorporating DCF mechanisms into terrestrial carbon models could improve the representation of microbial carbon inputs and their feedbacks to soil carbon dynamics.