Abstract
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 NH4+ 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.
Highlights
The tremendous increase in synthetic nitrogen (N) fertilizer uses has greatly promoted crop yields in the US since the early 20th century (Cao et al, 2018; Erisman et al, 2008)
We find that NH3 emissions from synthetic N fertilizer in the US remained less than 41 Gg N yr−1 before 1950 and sharply increased to 469 Gg N yr−1 in 1981, followed by a slower rise to 641 Gg N yr−1 by 2015 (Fig. 1a)
This study comprehensively examined the spatiotemporal patterns of NH3 emission owing to the changes in temperature, cropland area, rotation, and N fertilizer management in the US from 1900 to 2015
Summary
The tremendous increase in synthetic nitrogen (N) fertilizer uses has greatly promoted crop yields in the US since the early 20th century (Cao et al, 2018; Erisman et al, 2008). 5 %–9 % of the N applied was lost to the atmosphere through ammonia (NH3) volatilization (0.5–1 Tg N annually) across the US at the beginning of this century, which lowered the N use efficiency (NUE) of crops and caused numerous environmental issues (Bouwman et al, 2002; Cassman et al, 2002; Lu et al, 2019; Tilman et al, 2002). Nationwide, synthetic Nfertilizer-induced NH3 volatilization, contributing to 15 %– 30 % of annual total NH3 emission, has been identified as the second contributor to atmospheric NH3 only next to livestock production (Park et al, 2004; Paulot et al, 2014; Reis et al, 2009; US EPA, 2020). To quantify fertilizer-derived NH3 emission over space and time is essential in assessing agricultural N budget and improving
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