<p>The increasing demands of food and biofuel have promoted century-long cropland expansion and nitrogen (N) fertilizer enrichment in the United States. However, the role of such long-term human activities in influencing the spatiotemporal patterns of Ammonia (NH<sub>3</sub>) emission remains poorly understood. Based on an empirical model including climate, soil properties, N fertilizer management, and cropland distribution history, we have quantified monthly fertilizer-induced NH<sub>3</sub> emission across the contiguous U.S. from 1900 to 2015. Our results show that N fertilizer-induced NH<sub>3</sub> emission in the U.S. has increased from < 50 Gg N yr<sup>−1</sup> before the 1960s to 640 Gg N yr<sup>−1</sup> in 2015, for which corn and spring wheat planting is the dominant contributor. Meanwhile, urea-based fertilizers gradually grew to the largest NH<sub>3</sub> emitter and accounted for 78 % of the total increase during 1960–2015. Geospatial analysis reveals that hotspots of NH<sub>3</sub> emission have shifted from the central U.S. to the northwestern U.S. from 1960 to 2015. The increasing NH<sub>3</sub> emissions in the northwestern U.S has been found to closely correlate to the elevated wet NH<sub>4</sub><sup>+</sup> deposition in this region over the last three decades. This study shows that April, May, and June account for the majority of NH<sub>3</sub> emission in a year. Interestingly, the peak emission month has shifted from June to April since the 1960s. Our results imply that the northwestward corn and spring wheat expansion and growing urea-based fertilizer uses have dramatically altered the spatial pattern and temporal dynamics of NH<sub>3</sub> emission, impacting air pollution and public health in the U.S.</p>