Northwestward Cropland Expansion and Growing Urea-BasedFertilizer Use Enhanced NH 3 Emission Loss in the Contiguous UnitedStates

Abstract. 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 (NH3) 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 NH3 emission across the contiguous U.S. from 1900 to 2015. Our results show that N fertilizer-induced NH3 emission in the U.S. has increased from