AbstractDenitrification and dissimilatory nitrate reduction to ammonium (DNRA) are two microbial processes competing for nitrate and organic carbon (COD). Their competition has great implications for nitrogen loss, conservation, and greenhouse gas emissions. Nevertheless, a comprehensive and mechanistic understanding of the governing factors for this competition is still lacking. We applied the resource-ratio theory and verified it with competition experiments of denitrification and DNRA reported in the literature. Based on this theory, we revealed how COD/N ratio, influent resource concentrations, dilution rate, and stoichiometric and kinetic parameters individually and collectively define the boundaries for different competition outcomes in continuous cultures. The influent COD/N ratio alone did not drive competition outcome as the boundary COD/N ratio for different competition outcomes changed significantly with influent resource concentrations. The stoichiometry of the two processes was determinative for the boundaries, whereas the affinity for the resources (Ks), maximum specific growth rate (μmax) of the two species and the dilution rate had significant impacts as well but mainly at low influent resource concentrations (e.g., <100 μM nitrate). The proposed approach allows for a more comprehensive understanding of the parameters controlling microbial selection and explains apparently conflicting experimental results. The results from this model also provide testable hypotheses and tools for understanding and managing the fate of nitrate in ecosystems and for other species that compete for two resources.