Eutrophication and the disturbances which promote eutrophying condition may create environments favorable to the retention of nutrients, such as nitrogen (N), thus creating a positive feedback loop. This positive feedback loop is driven by 1) anoxia (due to oxygen consumption by bacteria) and 2) the increased availability of electron donors to heterotrophic microbial communities. The resulting alterations to the biogeochemical landscape dictate the degree to which bioavailable N is retained in an ecosystem, as NH₄, rather than converted to N₂O or N₂. To develop a framework for understanding the threshold at which competing microbial metabolisms will drive a net loss versus a net gain of inorganic N and provide a clear mechanism to explain the aforementioned positive feedback loop, we have used redox-constrained parameters (which may be more broadly applicable than species or location-specific parameters) to develop a microbial population model that mechanistically resolves the competition of dissimilatory nitrate reduction to ammonium (DNRA), an inorganic N retention pathway, versus denitrification and anaerobic ammonium oxidation (anammox), inorganic N loss pathways. These metabolisms compete for electron acceptors (NO₂) under varying electron donor (organic matter, OM) loading intensities. Our model demonstrates how we can understand the known response of DNRA and denitrification to varying OM and nitrite supply rate ratios from the underlying energetics, enabling a trait-based model. Therefore, our framework aids in improving our understanding of N cycling in environments where microbial community dynamics are largely unknown. In our model, net loss (i.e., N₂ production from denitrification) does not balance net production (i.e., NH₄ production from DNRA) when denitrification and DNRA communities reach coexistence or even when the DNRA functional group becomes the most abundant biomass. Instead, positive inorganic N production occurs when DNRA is able to reduce ~45% of available NO₂.
 

DNRA, denitrification, anammox, nitrite, organic matter