Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture

Agriculture is one of the oldest forms of technology on Earth. The cultivation of plants requires a terrestrial planet with active hydrological and carbon cycles and depends on the availability of nitrogen in soil. The technological innovation of agriculture is the active management of this nitrogen cycle by applying fertilizer to soil, at first through the production of manure excesses but later by the Haber–Bosch industrial process. The use of such fertilizers has increased the atmospheric abundance of nitrogen-containing species such as NH3 and N2O as agricultural productivity intensifies in many parts of the world. Both NH3 and N2O are effective greenhouse gases, and the combined presence of these gases in the atmosphere of a habitable planet could serve as a remotely detectable spectral signature of technology. Here we use a synthetic spectral generator to assess the detectability of NH3 and N2O that would arise from present-day and future global-scale agriculture. We show that present-day Earth abundances of NH3 and N2O would be difficult to detect, but hypothetical scenarios involving a planet with 30–100 billion people could show a change in transmittance of about 50%–70% compared to preagricultural Earth. These calculations suggest the possibility of considering the simultaneous detection of NH3 and N2O in an atmosphere that also contains H2O, O2, and CO2 as a technosignature for extraterrestrial agriculture. The technology of agriculture is one that could be sustainable across geologic timescales, so the spectral signature of such an "ExoFarm" is worth considering in the search for technosignatures.