Effective denitrification scales predictably with water residence time across diverse systems

Nitrogen provides a fundamental building block for life. However, some nitrogen species (e.g., NO~3~) cause water quality degradation and contribute to climatic warming as a greenhouse gas (e.g., N~2~O). Denitrification is a central process in the nitrogen cycle, transforming water-soluble nitrogen to gaseous nitrogen, thereby removing nitrogen from the system. The factors controlling denitrification are known, yet the general quantitative comparison of controls on denitrification remains a rich area of study. Understanding the relative importance of various controls over denitrification is critical given its biological importance, role in the Earth system, and the current perturbation of the global nitrogen cycle by human activity. We demonstrate that the 'effective' denitrification rate constant (first-order constant; k~e~ [T^-1^]) and advective water transport (mean residence time; [tau] [T]) scale inversely across diverse systems, ranging from hillslopes and groundwater to large lakes and estuaries. As a result, we observe a relatively constant nitrogen removal across nine orders of magnitude in transport time scales. A central question remains as to why k~e~ scales approximately linearly with [tau]. We suggest that the variability of k~e~ arises from hydrologic masking of an intrinsic denitrification rate constant (k~den~). A two-compartment model shows how k~e~ varies with the amount of advective water, and thus [tau], explaining how denitrification losses in the reactive zone of a system are controlled by mass transfer to that reactive zone. Our finding highlights mean residence time in the advective zone as a primary control on effective rates of denitrification, while other factors (e.g., nitrate and organic carbon concentrations) provide secondary controls. Furthermore, the relationship between k~e~ and [tau] demonstrates the strong coupling of the water and nitrogen cycles