Sources of nitrous oxide and the fate of mineral nitrogen in subarctic permafrost peat soils

Abstract

Abstract. Nitrous oxide (N2O) emissions from permafrost-affected terrestrial ecosystems have received little attention, largely because they have been thought to be negligible. Recent studies, however, have shown that there are habitats in the subarctic tundra emitting N2O at high rates, such as bare peat (BP) surfaces on permafrost peatlands. Nevertheless, the processes behind N2O production in these high-emission habitats are poorly understood. In this study, we established an in situ 15N-labeling experiment with two main objectives: (1) to partition the microbial sources of N2O emitted from BP surfaces on permafrost peatlands and (2) to study the fate of ammonium and nitrate in these soils and in adjacent vegetated peat (VP) surfaces showing low N2O emissions. Our results confirm the hypothesis that denitrification is mostly responsible for the high N2O emissions from BP. During the study period, denitrification contributed ∼ 79 % of the total N2O emissions from BP, whereas the contribution from ammonia oxidation was less (about 19 %). Both gross N mineralization and gross nitrification rates were higher in BP than in VP, with high C/N ratios and a low water content likely limiting N transformation processes and, consequently, N2O production in the latter soil type. Our results show that multiple factors contribute to high N2O production in BP surfaces on permafrost peatlands, with the most important factors being the absence of plants, an intermediate to high water content and a low C/N ratio, which all affect the mineral-N availability for soil microbes, including those producing N2O. The process understanding produced here is important for the development of process models that can be used to evaluate future permafrost–N feedbacks to the climate system.

The Net GHG Balance and Budget of the Permafrost Region (2000–2020) From Ecosystem Flux Upscaling

“…Although they occupy a small portion of the landscape (5%), Permafrost Bogs were the largest N 2 O emitters per unit area (Table S6, 645.14 μg N 2 O m −2 d −1 ) and their contribution to the regional N 2 O balance was 18%. The estimate for Permafrost Bogs includes emissions from barren peat surfaces, where vascular plants are absent ‐ surfaces previously identified as N 2 O hot spots in the Arctic due to ideal conditions for N 2 O production (Gil et al., 2022; Marushchak et al., 2011; Repo et al., 2009). A challenge remains regarding the mapping of Permafrost Bogs and barren ground and integration within land cover classifications.…”

Paleometagenomics reveals environmental microbiome response to vegetation changes in northern Siberia over the millennia

“…Similar to our observations, studies on modern treeline gradients showed a shift of soil nitrogen cycling along with vegetation, with higher inorganic nitrogen pools and microbial nitrogen cycling communities, both ammonia oxidizers and denitrifiers, in tundra compared to forest. The absence of trees supporting ectomycorrhizal nitrogen mining was identified as the factor promoting archaeal/bacterial‐driven inorganic nitrogen cycling (Clemmensen et al, 2021; Gil et al, 2022). Thus, our paleo‐reconstruction of plant‐microbiome dynamics and their functional consequences for nitrogen biogeochemical cycle finds full support in studies on modern environments, demonstrating to be a reliable new analytical approach with a tremendous potential to untangle complex interplays at ecosystem level.…”

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

“…On a global scale, N 2 O accounts for about 8% of the terrestrial denitrification flux, and thus has a higher yield than nitrification [193,201]. In permafrost-affected soils, denitrification is the main process of N 2 O emission, but nitrification can also be a major contributor (20% [202]) or even dominate ( [203], 86% [163]).…”

“…The bare peat soils in uplifted palsas and peat plateaus have been termed as Cryic Folic Histosol [246], and the surfaces are characterized by old peat material (age up to 6000 years) with a high degree of peat decomposition [243,244]. The bare peat surfaces have higher total labile carbon and nitrogen, lower C/N ratio, lower pH of about 3-4, higher bulk density, and lower phosphate content, unlike the surrounding vegetated peat soils (Fibric Histosols) [86,202,216,241,242,244,246,247]. In addition, the water table in the palsas and peat plateaus is lower than in the surrounding peatlands without permafrost, with the water content as water-filled pore space (WFPS) in the bare peat surfaces often at 60-70% [86,202,217], i.e., in the intermediate range favorable for N 2 O emission.…”