  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 = 10^12 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.
