Metabolic pathways of CO2 fixing microorganisms determined C-fixation rates in grassland soils along the precipitation gradient

CO2 fixing microorganisms (CFMs) play a crucial role in carbon (C) sequestration in vegetation restricted areas, e.g., under semiarid and arid conditions. The factors controlling the underlying pathways of the CO2 fixation by microorganisms living in soils remain unclear. Here, almost all genes responsible for the eight CO2 fixation pathways in semiarid soil CFMs communities were identified using metagenomic analysis: including the reductive citrate cycle (rTCA), dicarboxylate-hydroxybutyrate cycle (DC/4-HB), reductive pentose phosphate cycle (Calvin), 3-hydroxypropionate bicycle (3-HP), 3-hydroxypropionate/4-hydroxybutyrate (3-HP/4-HB), C4-dicarboxylic acid, CAM cycle, and reductive acetyl-CoA pathway (Wood-Ljungdahl pathway). By tracing the CO2 fixation flux via 13C labeling, it was shown that the CO2 fixation rates increased along the precipitation gradient. The rTCA and 3-HP pathways for CO2 fixing microorganisms were closely associated with 13C incorporation into the soil organic matter under high mean annual precipitation (MAP) (400–600 mm), whereas the Calvin cycle played a vital role in soils under low MAP (<400 mm) conditions. The abundance of the key genes within the C fixing pathways showed that the microbial C accumulation in soils was mainly influenced by the MAP. In semi-arid to semi-humid grassland soils, where CO2 fixation by CFMs provided about 8.1–27 mg C m−2 day−1 input into the ecosystem, we demonstrated that the rTCA, Calvin, and 3-HP cycle were vital to this essential pathway of C sequestration.