Evaluating the Plausible Range of N2O Biosignatures on Exo-Earths: An Integrated Biogeochemical, Photochemical, and Spectral Modeling Approach

Nitrous oxide (N₂O)-a product of microbial nitrogen metabolism-is a compelling exoplanet biosignature gas with distinctive spectral features in the near- and mid-infrared, and only minor abiotic sources on Earth. Previous investigations of N₂O as a biosignature have examined scenarios using Earthlike N₂O mixing ratios or surface fluxes, or those inferred from Earth's geologic record. However, biological fluxes of N₂O could be substantially higher, due to a lack of metal catalysts or if the last step of the denitrification metabolism that yields N₂ from N₂O had never evolved. Here, we use a global biogeochemical model coupled with photochemical and spectral models to systematically quantify the limits of plausible N₂O abundances and spectral detectability for Earth analogs orbiting main-sequence (FGKM) stars. We examine N₂O buildup over a range of oxygen conditions (1%-100% present atmospheric level) and N₂O fluxes (0.01-100 teramole per year; Tmol = 10¹² mole) that are compatible with Earth's history. We find that N₂O fluxes of 10 [100] Tmol yr^-1 would lead to maximum N₂O abundances of ~5 [50] ppm for Earth-Sun analogs, 90 [1600] ppm for Earths around late K dwarfs, and 30 [300] ppm for an Earthlike TRAPPIST-1e. We simulate emission and transmission spectra for intermediate and maximum N₂O concentrations that are relevant to current and future space-based telescopes. We calculate the detectability of N₂O spectral features for high-flux scenarios for TRAPPIST-1e with JWST. We review potential false positives, including chemodenitrification and abiotic production via stellar activity, and identify key spectral and contextual discriminants to confirm or refute the biogenicity of the observed N₂O.