In the 20th century, industrial nitrogen fixing by the Haber-Bosch process came to dominate the nitrogen cycle of Earth, allowing its human population to grow from 1.6 billion in 1900 to around 8 billion today. Around 50% of the nitrogen atoms in humans alive today were industrially fixed from nitrogen in the atmosphere. This leaves a signature in the atmosphere which aliens could detect from afar as the Earth transits the Sun. Perhaps we could detect alien civilisations with industrial agriculture the same way.