Wetland soils are the greatest source of nitrous oxide (N₂O), a critical greenhouse gas and ozone depleter released by microbes. Yet, microbial players and processes underlying the N₂O emissions from wetland soils are poorly understood. Using in situ N₂O measurements and by determining the structure and potential functional of microbial communities in 645 wetland soil samples globally, we examined the potential role of archaea, bacteria, and fungi in nitrogen (N) cycling and N₂O emissions. We show that N₂O emissions are higher in drained and warm wetland soils, and are correlated with functional diversity of microbes. We further provide evidence that despite their much lower abundance compared to bacteria, nitrifying archaeal abundance is a key factor explaining N₂O emissions from wetland soils globally. Our data suggest that ongoing global warming and intensifying environmental change may boost archaeal nitrifiers, collectively transforming wetland soils to a greater source of N₂O.

Nitrous oxide (N₂O) is a powerful greenhouse gas and the main driver of stratospheric ozone depletion. Since soils are the largest source of N₂O, predicting soil response to changes in climate or land use is central to understanding and managing N₂O. Here we find that N₂O flux can be predicted by models incorporating soil nitrate concentration (NO₃⁻), water content and temperature using a global field survey of N₂O emissions and potential driving factors across a wide range of organic soils. N₂O emissions increase with NO₃⁻ and follow a bell-shaped distribution with water content. Combining the two functions explains 72% of N₂O emission from all organic soils. Above 5 mg NO₃⁻-N kg⁻¹, either draining wet soils or irrigating well-drained soils increases N₂O emission by orders of magnitude. As soil temperature together with NO₃⁻ explains 69% of N₂O emission, tropical wetlands should be a priority for N₂O management.

Peatland restoration may provide a potential after-use option to mitigate the negative climate impact of abandoned peat extraction areas; currently, however, knowledge about restoration effects on the annual balances of carbon (C) and greenhouse gas (GHG) exchanges is still limited. The aim of this study was to investigate the impact of contrasting mean water table levels (WTLs) on the annual C and GHG balances of restoration treatments with high (Res_H) and low (Res_L) WTL relative to an unrestored bare peat (BP) site. Measurements of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) fluxes were conducted over a full year using the closed chamber method and complemented by measurements of abiotic controls and vegetation cover. Three years following restoration, the difference in the mean WTL resulted in higher bryophyte and lower vascular plant cover in Res_H relative to Res_L. Consequently, greater gross primary production and autotrophic respiration associated with greater vascular plant cover were observed in Res_L compared to Res_H. However, the means of the measured net ecosystem CO₂ exchanges (NEE) were not significantly different between Res_H and Res_L. Similarly, no significant differences were observed in the respective means of CH₄ and N₂O exchanges. In comparison to the two restored sites, greater net CO₂, similar CH₄ and greater N₂O emissions occurred in BP. On the annual scale, Res_H, Res_L and BP were C sources of 111, 103 and 268 g C m⁻² yr⁻¹ and had positive GHG balances of 4.1, 3.8 and 10.2 t CO₂ eq ha⁻¹ yr⁻¹, respectively. Thus, the different WTLs had a limited impact on the C and GHG balances in the two restored treatments 3 years following restoration. However, the C and GHG balances in Res_H and Res_L were considerably lower than in BP due to the large reduction in CO₂ emissions. This study therefore suggests that restoration may serve as an effective method to mitigate the negative climate impacts of abandoned peat extraction areas.

Summary: Trees are known to emit methane (CH₄) and nitrous oxide (N₂O), with tropical wetland trees being considerable CH₄ sources. Little is known about CH₄ and especially N₂O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH₄ and N₂O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400‐yr‐old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH₄ and N₂O, as were cryptogams, which seemed to be co‐responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH₄. Their N₂O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH₄ and N₂O. The volcanic tropical lowland rain forest appears to be an important CH₄ sink, as well as a possible N₂O sink.