Isotopic composition of N2O emissions from a permafrost peatland: a laboratory study using three different analytical techniques

Nitrous oxide (N2O) is a very potent greenhouse gas, and it is also involved in stratospheric ozone destruction. It is primarily produced by microbial processes such as nitrification and denitrification. Emissions of N2O from permafrost-affected soils have only recently been discovered but are of particular concern as climate change accelerates permafrost thaw and also N2O production. Nevertheless, mechanisms underlaying N2O emissions form permafrost-affected soils remain largely unresolved. Therefore, better understanding of N2O production and consumption processes is urgently needed, and isotope tools are critical for advancing this knowledge.   

Advances in isotopic laser spectroscopy, such as cavity ring-down spectroscopy (CRDS), have enabled real-time quantification of N2O isotopic ratios, offering a powerful tool to study isotope signals of N2O and microbial pathways. Here, an incubation experiment was conducted with soils collected from a permafrost peatland (bare and vegetated). Each of them was subjected to variable water holding capacities (WHC) ranging from 20% to 100%, since water availability is a primary controlling factor on N2O fluxes from soils. Incubations took place at the standard temperature of 15°C.  

Additionally, the study compared three methods for determining the isotopic signature of N2O sources. In the first method, discrete gas samples were collected into glass vials over the incubation period and later analyzed offline using the Keeling plot to derive the isotopic composition. For the second method, endpoint sampling, gas samples were collected at the end of the incubation into gas bags and analyzed to directly determine the isotopic signature of the accumulated N2O. The third method involved real-time isotopic measurements, connected directly to the incubation bottles via a multiplexer. The inverse Keeling plot was then used to derive the isotopic signature. All isotope analysis of N2O were done using the Picarro G5131-i isotopic N2O analyzer.  

Reliable isotopic data could only be obtained when the N2O flux flux exceeded the equivalent to 3 ppb per hour, which was rarely achieved. In the few cases, where fluxes were higher, the isotope signature of N2O indicated that denitrification was the main pathway at all moisture levels. The traditional Keeling plot approach was the most reliable method to determine the isotope source, but the inverse Keeling plot approach can be developed and offers, similar to the gas bag method, practical advantages. We discuss pros and cons of each method and ways to improve precision and reliability of the isotopic measurements in case of high and low fluxes.