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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 (NH 3) 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 NH 3 emission across the contiguous US from 1900 to 2015. Our results show that N-fertilizer-induced NH 3 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 NH 3 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 NH 3 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 NH 3 emissions have shifted from the central US to the Northern Great Plains from 1960 to 2015. The increasing NH 3 emissions in the Northern Great Plains have been found to closely correlate to the elevated NH 4+ deposition in this region over the last 3 decades. This study shows that April, May, and June account for the majority of NH 3 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 NH 3 emission, impacting air pollution and public health in the US. [ABSTRACT FROM AUTHOR]

• We synthesized county-level crop N budget and climate stress data during 1981-2016. • The U.S. corn N surplus dynamics are strongly regulated by climatic stresses. • A 51% increase in U.S. corn N surplus was associated with extremely hot conditions. • N surplus increases of 47% (dry) and 20% (wet) were related to extreme water stress. • The Midwest and the Northern Great Plains are hotspots of climate-driven N surplus. Increasing extreme climate conditions present significant threats to crop nitrogen (N) management and environmental conservation in the United States. Previous studies have examined the impacts of extreme climate on crop production but often overlooked its impacts on N loss from agricultural areas to the environment. Here we examined the relationship between county-level N surplus (the part of N that is not recovered by crops) and extreme climate conditions for corn in the U.S. during 1981–2016. We adopted multi-source long-term datasets of corn N budget and growing season (defined from May to August) maximum temperature and precipitation for each corn-planting county. Compared with the long-term N surplus trend, we found an increase of 51% and 47% in corn N surplus associated with extremely high temperature and extremely low precipitations, respectively. A moderate N surplus increase (20%) was associated with extremely high precipitations, while an N surplus decrease of 14% was shown to be related to extremely low temperatures. Across the U.S., the Midwest and the Northern Great Plains were identified as N surplus hotspots when extreme climate conditions occur. As the major corn-planting region, the Midwest accounted for 35% of the national extreme county-year samples but yielded 68% of the national total N surplus associated with extreme climate conditions. Our results highlighted the urgency of understanding the impacts of extreme climate conditions on crop nutrient losses and identifying the effective intervention practices to adapt to more frequent climate extremes in the future. [ABSTRACT FROM AUTHOR]

• A two-layer model was developed in TEM-HD to model C flux of mesic grasslands. • Elevated CO 2 has a fertilization effect in boosting the NPP in grassland ecosystems. • Warming is more harmful to the NPP in grasslands adapted to a warmer climate. • Grasslands likely experience a variable C exchange under a mild warming scenario. • Severe warming likely turns grasslands to net C sources by the end of the century. Atmospheric CO 2 increased in the 20th century and is expected to continue to do so in the 21st century, with resultant warming. Even so, the effects of these changes on the ecological systems, such as carbon sequestration in grassland ecosystems, are still poorly understood. To improve our understanding of the carbon balance, we developed a two-layer soil hydrology model for Terrestrial Ecosystem Model-Hydro Daily to simulate the carbon fluxes of moist grasslands more accurately. Using the outputs of two Representative Concentration Projection scenarios (RCP4.5 and 8.5) from five Coupled Model Inter-comparison Project Phase 5 climate models, we investigated if projected warming and rising atmospheric CO 2 could stimulate net primary production (NPP), net ecosystem productivity (NEP), and ecosystem respiration of two highly productive grassland sites in the central Great Plains (USA) in the 21st century. Our study shows that elevated atmospheric CO 2 has a fertilization effect in boosting NPP in grassland ecosystems, with a sensitivity of 0.53 gC m−2 yr−1 and 1.06 gC m−2 yr−1 under the RCP4.5 and RCP8.5 climate scenarios, respectively. Warming is more harmful to NPP in the grassland adapted to a warmer climate. Under the RCP4.5 scenario, both grassland sites likely experience a variable net ecosystem carbon exchange. However, the RCP8.5 scenario and accompanying severe warming would turn both grassland sites to net carbon sources by the end of the century, decreasing NEP by 0.97 gC m−2 yr−1 at the warmer site and by 0.96 gC m−2 yr−1 at the cooler site, driven by increased respiration and limited plant growth. [ABSTRACT FROM AUTHOR]