Autotrophs are widely distributed in soils of semiarid farmlands. However, due to limited research, the species and characteristics of the carbon (C) fixation capacity of photoautotrophs and chemoautotrophs remain poorly understood. This knowledge gap significantly hinders our understanding of farmland soil C dynamics and fixation potential. In this study, we identified the photoautotrophic and chemoautotrophic microbial species in a semiarid farmland soil and assessed their CO2 fixation capacity using metagenomics and in-situ 13CO2 tracing techniques. The results revealed that in the semiarid farmland soil, photoautotrophs (such as Chloroflexi bacterium CSP1-4 and Sphingomonas sp. URHD0057) and chemoautotrophs (such as Solirubrobacter sp. URHD0082 and Gaiella sp. SCGC AG-212-M14) accounted for approximately 13.6% and 86.4% of all autotrophs, respectively. Both photoautotrophs and chemoautotrophs were found to fix C at an overall rate of approximately 5.961 mg C m-2 d-1. Further analysis revealed that both autotrophs exhibited functional redundancy in C fixation, which may enhance their environmental adaptability. The unit abundance microbial C fixation rate of photoautotrophs was approximately 1.5 times that of chemoautotrophs. These findings underscore that atmospheric C transformation through photoautotrophs and chemoautotrophs occurs at a substantial rate in soils of semiarid farmlands, thereby broadening our understanding of C fixation characteristics and the ecological significance of soil autotrophs in semiarid farmland ecosystems.