In contrast to the well-recognized permafrost carbon (C) feedback to climate change, the fate of permafrost nitrogen (N) after thaw is poorly understood. According to mounting evidence, part of the N liberated from permafrost may be released to the atmosphere as the strong greenhouse gas (GHG) nitrous oxide (N2O). Here, we report post-thaw N2O release from late Pleistocene permafrost deposits called Yedoma, which store a substantial part of permafrost C and N and are highly vulnerable to thaw. While freshly thawed, unvegetated Yedoma in disturbed areas emit little N2O, emissions increase within few years after stabilization, drying and revegetation with grasses to high rates (548 (133–6286) μg N m−2 day−1; median with (range)), exceeding by 1–2 orders of magnitude the typical rates from permafrost-affected soils. Using targeted metagenomics of key N cycling genes, we link the increase in in situ N2O emissions with structural changes of the microbial community responsible for N cycling. Our results highlight the importance of extra N availability from thawing Yedoma permafrost, causing a positive climate feedback from the Arctic in the form of N2O emissions.

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Soils are important sources of nitric oxide (NO) and nitrous acid (HONO) in the atmosphere. These nitrogen (N)-containing gases play a crucial role in atmospheric chemistry and climate at different scales because of reactions modulated by NO and hydroxyl radicals (OH), which are formed via HONO photolysis. Northern permafrost soils have so far remained unexplored for HONO and NO emissions despite their high N stocks, capacity to emit nitrous oxide (N2O), and enhancing mineral N turnover due to warming and permafrost thawing. Here, we report the first HONO and NO emissions from high-latitude soils based on measurements of permafrost-affected subarctic peatlands. We show large HONO (0.1 to 2.4 µg N m-2 h-1) and NO (0.4 to 59 µg N m-2 h-1) emissions from unvegetated peat surfaces, rich with mineral N, compared to low emissions (≤ 0.2 µg N m-2 h-1 for both gases) from adjacent vegetated surfaces (experiments with intact peat cores). We observed HONO production under highly variable soil moisture conditions from dry to wet. However, based on complementary slurry experiments, HONO production was strongly favored by high soil moisture and anoxic conditions. We suggest urgent examination of other Arctic landscapes for HONO and NO emissions to better constrain the role of these reactive N gases in Arctic atmospheric chemistry.

Nitrous acid (HONO) photolysis produces hydroxyl radicals—a key atmospheric oxidant. Soils are strong HONO emitters, yet HONO production pathways in soils and their relative contributions are poorly constrained. Here, we conduct 15N tracer experiments and isotope pool dilution assays on two types of agricultural soils in Finland to determine HONO emission fluxes and pathways. We show that microbial processes are more important than abiotic processes for HONO emissions. Microbial nitrate reduction (denitrification) considerably exceeded ammonium oxidation as a source of nitrite—a central nitrogen pool connected with HONO emissions. Denitrification contributed 97% and 62% of total HONO fluxes in low and high organic matter soil, respectively. Microbial ammonium oxidation only produced HONO in high organic matter soil (10%). Our findings indicate that microbial nitrate reduction is an important HONO production pathway in aerobic soils, suggesting that terrestrial ecosystems favouring it could be HONO emission hotspots, thereby influencing atmospheric chemistry.

<p><strong>Background</strong></p><p>Agricultural soils produce large quantities of greenhouse gases (GHG). Especially organic soils, such as peat, can act as a source of carbon dioxide (CO<sub>2</sub>) and nitrous oxide (N<sub>2</sub>O) when the natural water table height is lowered for agricultural use, allowing aerobic decomposition of the previously waterlogged organic matter. While organic soils, such as peat, make up approximately 13% of the total arable land area in Finland, CO2 emissions from cultivated peat constitute 40% of the total CO<sub>2,</sub> and 22% of the N<sub>2</sub>O emissions from agriculture. These emissions are the result of microbial activity related to carbon and nitrogen cycles, and according to current knowledge microbial activity is regulated by the pH and electrical conductivity of the soil. Soil amendments such as lime and wood ash are used to improve the alkalinity of cultivated soil and may influence microbial activity. Earlier experiments have also shown that wood ash addition can decrease the N<sub>2</sub>O emissions from cultivated peat. Researching the extent to which it is possible to mitigate these GHG emissions with soil amendments is of vital importance in order to build sustainable land use practices and guidelines for agricultural use of peatlands.</p><p> </p><p><strong>Method</strong></p><p>In our research we aim to study the effects of different soil amendments on GHG emissions from cultivated peatlands. The soil amendments that we study are wood ash, lime (calcium carbonate, CaCO<sub>3</sub>), gypsum (CaSO<sub>4</sub>* 2H<sub>2</sub>O), and biochar. The soils we use are collected from four different cultivated peatland sites, and the effects were studied in bottle and core incubation experiments where the GHG emission rates were measured weekly. The soil was also sampled, and samples flash-frozen, before and after the incubation to allow for DNA and RNA extraction, for purposes of determining the soil microbe community structure and activity. We determine the soil microbial community by amplifying 16S rRNA-gene from the extracted DNA and sequencing the amplified DNA with MiSeq equipment. To further study the community structure and activity we determine the copy numbers of selected enzyme-coding genes (amoA, nirK, nirS, narG, nrfA) related to nitrogen cycling from both the extracted DNA and RNA using Quantitative PCR, and Quantitative Reverse Transcription PCR methods respectively. In addition to measuring the GHG emissions, we also measure the nitrous acid (HONO) and nitric oxide (NO) emissions from the soils during the experiment. Nitrous acid is precursor of atmospheric NO that depletes ozone, and hydroxyl radicals (OH) that can oxidize atmospheric methane (CH<sub>4</sub>). Based on our initial results from the core incubations, we are also planning a follow up field experiment.</p>