Orchard soils exhibit the highest microbial CO2 fixation potential and contribution to soil organic carbon across land-use types in Qaidam Basin

Soil organic carbon content varies across different land-use types. Elucidating CO 2 -fixing microorganisms and their fixation potential is pivotal for accurately evaluating and improving soil carbon sinks and enriching the theory of microbial carbon sequestration. However, the CO 2 fixation potential, driving factors and mechanism of soil CO 2 -fixing microorganisms in different land-use types are still unclear. This study investigated the CO 2 fixation potential (CFP) and sequestration mechanism in 0–10 cm topsoil (36 soil samples) of typical grassland (GL), facility farmland (FAL), farmland (FL) and orchard (OD) in the Qaidam Basin by 13 CO 2 isotope tracer incubation method (61 d), metagenomic sequencing and partial least squares method. The 13 CO 2 -fixing rates (26.7 mg 13 C m −2 d −1 ) in OD were 2.26, 1.24 and 1.31 times those of GL, FAL and FL, respectively ( p < 0.05). Correspondingly, 160 unique CO 2 -fixing species were observed in OD, mainly including methanogenic archaeon ISO4-H5, Leptospira alstonii , Candidatus Saccharibacteria bacterium GW2011 GWA2 46 10 and Bacillus encimensis , with few unique species in other soils. GL showed higher relative abundances of the Calvin (8.78 %) and C4-Dicarboxylic acid (3.89 %) cycles ( p < 0.05). CFP was directly affected by metabolic pathways and soil properties, and indirectly by microbial communities ( p < 0.05), with ammonium ion, total nitrogen, and particulate organic carbon as key determinants. In conclusion, CFP can be improved by regulating soil physical-chemical factors. Our findings clarify the structural and functional variations of CO 2 -fixing microbial communities across land uses, offering theoretical support for exploring novel CO 2 -fixing microbes and optimizing CO 2 fixation capacity. • Orchard soils had the highest CO 2 fixation potential. • Orchard soils contained 160 unique CO 2 -fixing microbial species. • Grassland soils contained the metabolic pathway with the highest relative abundance. • Particulate organic carbon was the main factor affecting CO 2 fixation potential. • CO 2 fixation potential can be improved by regulating soil physical-chemical factors.