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Abstract The response of highly productive croplands at northern mid-latitudes to climate change is a primary source of uncertainty in the global carbon cycle, and a concern for future food production. We present a decadal time series (2007 to 2019) of hourly CO 2 concentration measured at a very tall tower in the United States Corn Belt. Analyses of this record, with other long-term data in the region, reveal that warming has had a positive impact on net CO 2 uptake during the early crop growth stage, but has reduced net CO 2 uptake in both croplands and natural ecosystems during the peak growing season. Future increase in summer temperature is projected to reduce annual CO 2 sequestration in the Corn Belt by 10–20%. These findings highlight the dynamic control of warming on cropland CO 2 exchange and crop yields and challenge the paradigm that warming will continue to favor CO 2 sequestration in northern mid-latitude ecosystems. 

Atmospheric ammonia (NH₃) has increased dramatically as a consequence of the production of synthetic nitrogen (N) fertilizer and proliferation of intensive livestock systems. It is a chemical of environmental concern as it readily reacts with atmospheric acids to produce fine particulate matter and indirectly contributes to nitrous oxide (N₂O) emissions. Here, we present the first tall tower observations of NH₃within the U.S. Corn Belt for the period April 2017 through December 2018. Hourly average NH₃mixing ratios were measured at 100 and 56 m above the ground surface and fluxes were estimated using a modified gradient approach. The highest NH₃mixing ratios (>30 nmol mol⁻¹) occurred during early spring and late fall, coinciding with the timing of fertilizer application within the region and the occurrence of warm air temperatures. Net ecosystem NH₃exchange was greatest in spring and fall with peak emissions of about +50 nmol m⁻² s^−l. Annual NH₃emissions estimated using state‐of‐the‐art inventories ranged from 0.6 to 1.4 × the mean annual gross tall tower fluxes (+2.1 nmol m⁻² s⁻¹). If the tall tower observations are representative of the Upper Midwest and broader U.S. Corn Belt regions, the annual gross emissions were +720 Gg NH₃‐N y⁻¹and +1,340 Gg NH₃‐N y⁻¹, respectively. Finally, considering the N₂O budget over the same region, we estimated total reactive N emissions (i.e., N₂O + NH₃) of approximately 1,790 Gg N y⁻¹from the U.S. Corn Belt, representing ~23% of the current annual new N input.