Foliar N₂O emissions constitute a significant source to atmosphere

Nitrous oxide (N2O) is a potent greenhouse gas and causes stratospheric ozone depletion. While the emissions of N2O from soil are widely recognized, recent research has shown that terrestrial plants may also emit N2O from their leaves under controlled laboratory conditions. However, it is unclear whether foliar N2O emissions are universal across varying plant taxa, what the global significance of foliar N2O emissions is, and how the foliage produces N2O in situ. Here we investigated the abilities of 25 common plant taxa, including trees, shrubs and herbs, to emit N2O under in situ conditions. Using N-15 isotopic labeling, we demonstrated that the foliage-emitted N2O was predominantly derived from nitrate. Moreover, by selectively injecting biocide in conjunction with the isolating and back-inoculating of endophytes, we demonstrated that the foliar N2O emissions were driven by endophytic bacteria. The seasonal N2O emission rates ranged from 3.2 to 9.2 ng N2O-N g(-1) dried foliage h(-1). Extrapolating these emission rates to global foliar biomass and plant N uptake, we estimated global foliar N2O emission to be 1.21 and 1.01 Tg N2O-N year(-1), respectively. These estimates account for 6%-7% of the current global annual N2O emission of 17 Tg N2O-N year(-1), indicating that in situ foliar N2O emission is a universal process for terrestrial plants and contributes significantly to the global N2O inventory. This finding highlights the importance of measuring foliar N2O emissions in future studies to enable the accurate assigning of mechanisms and the development of effective mitigation.