Hot moment of N 2 O emissions in seasonally frozen peatlands.

Since the start of the Anthropocene, northern seasonally frozen peatlands have been warming at a rate of 0.6 °C per decade, twice that of the Earth's average rate, thereby triggering increased nitrogen mineralization with subsequent potentially large losses of nitrous oxide (N 2 O) to the atmosphere. Here we provide evidence that seasonally frozen peatlands are important N 2 O emission sources in the Northern Hemisphere and the thawing periods are the hot moment of annual N 2 O emissions. The flux during the hot moment of thawing in spring was 1.20 ± 0.82 mg N 2 O m -2 d -1 , significantly higher than that during the other periods (freezing, -0.12 ± 0.02 mg N 2 O m -2 d -1 ; frozen, 0.04 ± 0.04 mg N 2 O m -2 d -1 ; thawed, 0.09 ± 0.01 mg N 2 O m -2 d -1 ) or observed for other ecosystems at the same latitude in previous studies. The observed emission flux is even higher than those of tropical forests, the World's largest natural terrestrial N 2 O source. Furthermore, based on soil incubation with 15 N and 18 O isotope tracing and differential inhibitors, heterotrophic bacterial and fungal denitrification was revealed as the main source of N 2 O in peatland profiles (0-200 cm). Metagenomic, metatranscriptomic, and qPCR assays further revealed that seasonally frozen peatlands have high N 2 O emission potential, but thawing significantly stimulates expression of genes encoding N 2 O-producing protein complexes (hydroxylamine dehydrogenase (hao) and nitric oxide reductase (nor)), resulting in high N 2 O emissions during spring. This hot moment converts seasonally frozen peatlands into an important N 2 O emission source when it is otherwise a sink. Extrapolation of our data to all northern peatland areas reveals that the hot moment emissions could amount to approximately 0.17 Tg of N 2 O yr -1 . However, these N 2 O emissions are still not routinely included in Earth system models and global IPCC assessments.