Ammonia gas, which contributes to air pollution, is emitted from soils and combustion. In regions with distinct dry and rainy seasons, the first rainfall events each year trigger biogeochemical activity in soils. We used satellite observations of the atmosphere over the African Sahel savanna ecosystem to show that increases in soil moisture at the onset of the rainy season are responsible for large pulsed emissions of ammonia equal to roughly a fifth of annual ammonia emissions from the region