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

Abstract

Soil organic carbon content varies across different land-use types. Elucidating CO₂-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 CO₂ fixation potential, driving factors and mechanism of soil CO₂-fixing microorganisms in different land-use types are still unclear. This study investigated the CO₂ 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 ¹³CO₂ isotope tracer incubation method (61 d), metagenomic sequencing and partial least squares method. The ¹³CO₂-fixing rates (26.7 mg ¹³C 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 CO₂-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 CO₂-fixing microbial communities across land uses, offering theoretical support for exploring novel CO₂-fixing microbes and optimizing CO₂ fixation capacity.