Northwestward cropland expansion and growing urea-based fertilizer use enhanced NH3 emission loss in the contiguous United States

Abstract

The increasing demands of food and biofuel have promoted cropland expansion and nitrogen (N) fertilizer enrichment in the United States over the past century. 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 and time-series gridded datasets including temperature, soil properties, N fertilizer management, and cropland distribution history, we have quantified monthly fertilizer-induced NH3 emission across the contiguous US from 1900 to 2015. Our results show that N-fertilizer-induced NH3 emission in the US has increased from < 50 Gg N yr-1 before the 1960s to 641 Gg N yr-1 in 2015, for which corn and spring wheat are the dominant contributors. Meanwhile, urea-based fertilizers gradually grew to the largest NH3 emitter and accounted for 78 % of the total increase during 1960 2015. The factorial contribution analysis indicates that the rising N fertilizer use rate dominated the NH3 emission increase since 1960, whereas the impacts of temperature, cropland distribution and rotation, and N fertilizer type varied among regions and over periods. Geospatial analysis reveals that the hot spots of NH3 emissions have shifted from the central US to the Northern Great Plains from 1960 to 2015. The increasing NH3 emissions in the Northern Great Plains have been found to closely correlate to the elevated NHC 4 deposition in this region over the last 3 decades. This study shows that April, May, and June account for the majority of NH3 emission in a year. Interestingly, the peak emission month has shifted from May 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 NH3 emission, impacting air pollution and public health in the US.