Land use changes and nitrogen fertilizer application in the Yangtze River Delta region from 2000 to 2018 have led to reduced ammonia emissions from farmland ecosystems. The area of cultivated land decreased as urbanization accelerated, resulting in lower nitrogen fertilizer use and decreased ammonia emissions from both soil and fertilizer. Ammonia emissions from nitrogen fertilizer application decreased by 22%, while emissions from soil background decreased by 9% over the 20-year period.