Predicting where, and how much, rising freshwater nitrogen pollution leads to higher nitrous oxide (N2O) emissions is critical to improving global greenhouse gas budgets. N2O is produced as a reactive intermediate during microbial nitrogen cycling, yet concentrations of inorganic nitrogen species (e.g., NO3-) explain only ~30% of measured variability in stream N2O. Here I use empirical measurements of N2O over diurnal time periods across ‘atypical’ fluvial systems (agricultural drainage networks, sediment-laden streams, and algae-dominated braided rivers) to test the limits of proposed relationships between catchments, nitrogen pollution, and N2O emissions. The ratio of N2O (% saturation) to surface water NO3- (mg N l-1) ranged from 20 to 200 across these ten sites. Parallel measurements of diel dissolved oxygen suggest a metabolic driver of the partitioning of available inorganic nitrogen into N2O. Findings highlight the importance of accounting for the biological complexity underpinning stream N2O emissions, which were observed to alternatively increase and decrease during dark periods across these diverse waterways.