Hydrologic Connectivity Regulates Riverine N 2 O Sources and Dynamics.

Indirect nitrous oxide (N 2 O) emissions from streams and rivers are a poorly constrained term in the global N 2 O budget. Current models of riverine N 2 O emissions place a strong focus on denitrification in groundwater and riverine environments as a dominant source of riverine N 2 O, but do not explicitly consider direct N 2 O input from terrestrial ecosystems. Here, we combine N 2 O 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 N 2 O in a mesoscale river network within the U.S. Corn Belt. We find that N 2 O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N 2 O across the entire river network. The delivery of soil-produced N 2 O 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 N 2 O input implies an important climate-N 2 O feedback mechanism that may enhance riverine N 2 O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N 2 O emissions may result in significant underestimations.