Abstract
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.