The biogeochemical cycles of carbon (C) and nitrogen (N) are inextricably linked for a range of reactions. For coupled reactions such as denitrification to occur, however, solutes must be found together in space and time. Using the framework of concentration-discharge (c-Q) relationships, we examine the frequency of synchronous C and N export (i.e. identical c-Q behavior) across a river network using > 5 years of high-frequency sensor data. We demonstrate that across space and time the export of C and N to a river network is asynchronous 57% of the time. The probability of simultaneous export in largely forested watersheds demonstrates little temporal structure, while in more human-impacted watersheds, we observe the highest frequency of asynchronous c-Q behavior. We discuss the implications of synchronous c-Q behavior for solute flux estimation models and develop a theoretical framework for predicting where within a landscape we expect the probability of coupled C and N reactions to be greatest. By simultaneously comparing the variability in C and N c-Q relationships we develop an integrated framework for predicting synchronous export of solutes.
