Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N 2 O), a potent greenhouse gas; however, this has not been well quantified yet.

Here we examine how multiple environmental forcings have affected N 2 O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N 2 O-N yr -1 in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N 2 O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N 2 O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr -1 . This suggests a global emission factor of 0.051% for inland water N 2 O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories.

Nitrous oxide (N 2 O) is a potent greenhouse gas, with ~273 times the warming potential of carbon dioxide on a 100-year time horizon, and also contributes to stratospheric ozone destruction 1-3 . Nitrogen (N) processes in inland waters, as a critical component of the global N cycle, are gaining recognition for their important contribution to N 2 O emissions through nitrification and denitrification 4,5 . These emissions, expressed in carbon dioxide (CO 2 ) equivalents, will offset ~4% of the land carbon sink 6 . Several preceding studies have been dedicated to assessing the magnitude of N 2 O emissions from inland waters on regional and global scales 5,7,8 . However, the global estimates are still weakly constrained, particularly for lentic systems such as lakes and reservoirs.

Sizeable human activities have contributed to a notable increase in anthropogenic N loads that are transported from land to lentic systems, thereby playing a significant role in N 2 O emissions originating from these systems 8-10 . However, based only on sparse and unevenly distributed local measurements, most previous estimates on N 2 O emissions from lentic systems are varied by approximately four-fold (160.00-583.00 Gg N yr -1 ) 5,11,12 . Furthermore, human-induced N 2 O emission from lentic systems are implicitly incorporated, as the indirect agricultural N 2 O emissions, into the recent national N 2 O emission inventory from the United Nations Framework Convention on Climate Change (UNFCCC), which is calculated based on anthropogenic N additions and global mean emission factors 13,14 . Nevertheless, the use of constant and linear emission factors in emission inventory fails to capture the spatial variability of N 2 O emissions from lentic systems 8 and cannot dynamically attribute them