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Atmospheric concentrations of the powerful greenhouse gas nitrous oxide (N₂O) have increased dramatically over the last 100 years, and part of these emissions come from streams and rivers. N₂O production has been more carefully studied in soils, but runoff of reactive nitrogen, likely from fertilizer, influences lotic N₂O emissions. N₂O production and consumption are strongly microbially mediated and mostly involve oxidation and reduction of the reactive nitrogen species ammonia, nitrate, and nitrite. Of the four main pathways leading to N₂O production in soils and sediments, incomplete denitrification is likely the globally dominant N₂O generating pathway and is favored by elevated nitrate concentrations, suboxic conditions, and sufficient organic carbon to promote reduction. The two pathways that oxidize ammonia, nitrifier denitrification and nitrification, are favored with higher concentrations of dissolved …