Hydrologic Connectivity Regulates Riverine N₂O Sources and Dynamics

Indirect nitrous oxide (N₂O) emissions from streams and rivers are a poorly constrained term in the global N₂O budget. Current models of riverine N₂O emissions place a strong focus on denitrification in groundwater and riverine environments as a dominant source of riverine N₂O, but do not explicitly consider direct N₂O input from terrestrial ecosystems. Here, we combine N₂O isotope measurements and spatial stream network modeling to show that terrestrial-aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N₂O in a mesoscale river network within the U.S. Corn Belt. We find that N₂O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N₂O across the entire river network. The delivery of soil-produced N₂O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N₂O input implies an important climate-N₂O feedback mechanism that may enhance riverine N₂O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N₂O emissions may result in significant underestimations.