Inland Waters Increasingly Produce and Emit Nitrous Oxide.

Nitrous oxide (N 2 O) is a long-lived greenhouse gas and currently contributes ∼10% to global greenhouse warming. Studies have suggested that inland waters are a large and growing global N 2 O source, but whether, how, where, when, and why inland-water N 2 O emissions changed in the Anthropocene remains unclear. Here, we quantify global N 2 O formation, transport, and emission along the aquatic continuum and their changes using a spatially explicit, mechanistic, coupled biogeochemistry-hydrology model. The global inland-water N 2 O emission increased from 0.4 to 1.3 Tg N yr -1 during 1900-2010 due to (1) growing N 2 O inputs mainly from groundwater and (2) increased inland-water N 2 O production, largely in reservoirs. Inland waters currently contribute 7 (5-10)% to global total N 2 O emissions. The highest inland-water N 2 O emissions are typically in and downstream of reservoirs and areas with high population density and intensive agricultural activities in eastern and southern Asia, southeastern North America, and Europe. The expected continuing excessive use of nutrients, dam construction, and development of suboxic conditions in aging reservoirs imply persisting high inland-water N 2 O emissions.