Constraining the global permafrost nitrous oxide budget

Nitrous oxide (N2O) which is also known as the laughing gas is, after carbon dioxide and methane, the third most important greenhouse gas responsible for climate warming. It is produced mainly from soils as a result of microbial activity. Usually, high nitrous oxide emissions occur from agricultural soils, where the availability of mineral nitrogen is high because of nitrogen fertilisation and other management practices. Since the nitrogen cycling in cold permafrost soils is slow, they have previously been regarded as unimportant nitrous oxide sources. Based on accumulating evidence during the past years, however, this is not always true: nitrous oxide release has been found to be a common phenomenon in permafrost-affected soils. However, the magnitude of the emissions as well as factors controlling them remain poorly known. The project PERNO will fill this gap by researching deeply into fluxes and factors controlling N2O emission from permafrost soils. Therefore, we will incubate soils from permafrost regions in the laboratory and manipulate temperature and soil moisture to simulate climate change. Permafrost soils will be also thawn in the laboratory to simulate permafrost melting. We will monitor the emissions of N2O and other reactive and non-reactive nitrogen gases, such as NO and N2 over time with modern laser spectroscopic methods. Additionally, we will investigate accompanying changes in the microbial community and in soil parameters from the soils. Importantly, the stable isotope composition of nitrous oxide will be measured continuously in the laboratory to get valuable information on microbial pathways responsible for the N2O emissions. Nitrous oxide is produced in sols during nitrification and denitrification, and each pathways produces a unique stable isotope fingerprint in N2O. This information is important to characterize and better understand the N2O emissions. Stable isotope techniques have become integral parts in modern ecology and contribute to gain detailed insights into the nitrogen cycle. The stable isotope signature of N2O will be also used in the process-based models which are also employing in the project PERNO. We will use an isotope model (IsoTONE) to simulate the N2O fluxes and extrapolate them in space and time. Overall, PERNO aims to fill critical gaps in knowledge on N2O dynamics from permafrost soils and shed light on the importance of these emissions from permafrost soils in present and future climate. We hypothesize that N2O from permafrost soils forms a potentially significant positive feedback to climate change.

Permafrost soils store vast amounts of organic matter that becomes available for microbes when the soil warms and thaws. While carbon dioxide and methane emissions from these regions are already well studied, far less is known about nitrous oxide (N2O), a greenhouse gas roughly 300 times more potent than CO2. The PERNO project aimed to close this important knowledge gap by providing the first robust estimate of the global N2O budget from permafrost areas, and gaining more information on processes underlying the emissions. To achieve this, the project combined advanced laboratory experiments with state-of-the-art modelling, both of them supported by stable isotope measurements of N2O. A central step in the project was thus the use of a new isotope laser instrument that can measure this chemical "fingerprint" of N2O with high precision. This fingerprint allowed us to identify which microbial processes in the soil create N2O. Setting up this technique required careful calibration, the creation of standardized procedures, and collaboration with expert laboratories across Europe. We then incubated different soils from Arctic, alpine, and thawing permafrost landscapes under variable environmental conditions. The results clearly show that soil moisture strongly controls N2O emissions: intermediate to high water contents promote N2O production, whereas very wet soils often acted as sinks that consume N2O. Our isotope results also suggest that fungal denitrification is a major source of NO in certain hotspot areas, such as bare peat surfaces. To estimate regional and global emissions, we compiled a unique database of more than 1500 soil nitrogen-isotope measurements and created the first machine-learning-based "isoscape" map of permafrost regions. These data were used in IsoTONE, an atmospheric-ecosystem model that uses nitrogen isotope patterns to calculate N2O emissions. First model results indicate a release of stored nitrogen and N2O from permafrost soils due to thawing which is happening already now. Future warming and the release of more nitrogen during permafrost thaw could thus substantially amplify N2O emissions. Together, these results highlight that N2O emissions from permafrost regions represent a potentially important but previously underestimated climate feedback. PERNO provides the first comprehensive dataset and modelling framework needed to reliably quantify their role in the global nitrogen cycle and in future climate change.