Elucidating the Competition between Heterotrophic Denitrification and DNRA Using the Resource-Ratio Theory

Heterotrophic denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are two microbial processes competing for two shared resources, namely, 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 to study this competition and validated the theory with experimental data from continuous cultures reported in the literature. Based on this theory, we revealed that influent COD/N ratio alone was not sufficient to predict the competition outcome as the boundary values for different competition outcomes changed substantially with influent resource concentrations. The stoichiometry of the two processes was determinative for the boundaries, whereas the affinity for the shared 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.,