Hot moment of N2O emissions in seasonally frozen peatlands

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 (N₂O) to the atmosphere. Here we provide evidence that seasonally frozen peatlands are important N₂O emission sources in the Northern Hemisphere and the thawing periods are the hot moment of annual N₂O emissions. The flux during the hot moment of thawing in spring was 1.20 ± 0.82 mg N₂O m^-2 d^-1, significantly higher than that during the other periods (freezing, -0.12 ± 0.02 mg N₂O m^-2 d^-1; frozen, 0.04 ± 0.04 mg N₂O m^-2 d^-1; thawed, 0.09 ± 0.01 mg N₂O 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 N₂O source. Furthermore, based on soil incubation with ¹⁵N and ¹⁸O isotope tracing and differential inhibitors, heterotrophic bacterial and fungal denitrification was revealed as the main source of N₂O in peatland profiles (0-200 cm). Metagenomic, metatranscriptomic, and qPCR assays further revealed that seasonally frozen peatlands have high N₂O emission potential, but thawing significantly stimulates expression of genes encoding N₂O-producing protein complexes (hydroxylamine dehydrogenase (hao) and nitric oxide reductase (nor)), resulting in high N₂O emissions during spring. This hot moment converts seasonally frozen peatlands into an important N₂O 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 N₂O yr^-1. However, these N₂O emissions are still not routinely included in Earth system models and global IPCC assessments.