Soil inorganic carbon (SIC) represents the main soil carbon pool in drylands with a high geologic residence time for carbon sequestration. Recent studies have shown that SIC is not stable as previously supposed, and can be employed by certain microbes and transformed into organics in soils; however, this transformation remains largely unexplored. We performed in situ ¹³C tracing in desert bulk soil and employed metagenomics to predict the microbial metabolic processes associated with carbon transformation. The tracing data showed that the ¹³C signature profile in soil organic carbon (SOC) originated from SIC with a ¹³C-SOC content of 6.881 mg m^-2 during the feeding periods. Metagenomic analysis identified genes encoding enzymes related to microbial CO₂ and HCO₃^- fixation, accounting for 0.448% (based on Kyoto Encyclopedia of Genes and Genomes database) and 0.668% (based on Evolutionary genealogy of genes: Non-supervised Orthologous Groups database) of all ascertained genes. Our results confirmed that a considerable portion of the determined genes and taxa were responsible for heterotrophic fixation. The microbes involved in dark microbial fixation, particularly chemoautotrophic and heterotrophic pathways, were from a broad taxonomic range. Although the amount of SOC derived from the dark microbial fixation process was not assessed, the present study highlights a neglected carbon transformation process mediated by soil microbes in drylands and provides insights into carbon transformation of SIC to SOC in dryland soil.