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

Abstract

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.