Spatial Variation of Reactive Nitrogen Emissions From China's Croplands Codetermined by Regional Urbanization and Its Feedback to Global Climate Change

Reactive gaseous nitrogen (Ngr) emissions significantly affect Earth's climate system. Disagreement exists, however, over Ngr contributions to short- versus long-term climate forcing, from local to global scales and among different gaseous forms, including NH3, NOx, and N2O. Here, we provide a comprehensive inventory of Ngr from China's croplands based on a new bottom-up, mass flow-based approach integrated with fine-resolution agricultural activity data and nitrogen emission factors. We demonstrate that China's croplands emit about 8.87 Tg N to the atmosphere in 2014. Across different prefectures, Ngr emission per capita conforms to a "Kuznets curve," that is, first increases then decreases, along the gradient of increasing urbanization. Ngr emission per gross domestic productivity (GDP) decreases exponentially with increasing urbanization or per capita GDP. Furthermore, climate change impact analyses suggest that the global-scale warming effect of China's cropland N2O emissions dominate over local cooling effects ascribed to its NH3 and NOx emissions. Plain Language Summary Reactive gaseous nitrogen, sourced to the world's highest fertilizer application rates in China, imparts local cooling effects on short time scales. The effects are ultimately counterbalanced by nitrous oxide emissions over the long term, leading to substantial warming effects on the global climate system. We base our conclusion on the first-ever high-resolution assessment of reactive nitrogen emissions to the atmosphere from China's croplands. Owing to limited high-resolution reactive gaseous nitrogen emissions data, hitherto, large uncertainties have existed in the detailing the role of China's cropland reactive gaseous nitrogen emissions on climate forcing. This paucity of information poses a challenge for informing China's agricultural sustainability policies with regard to maintaining/increasing agricultural yields while reduces reactive gaseous nitrogen emissions and their warming potential. Our analyses provide essential information for further sustainable N management and the interactions between agriculture and climate change. We provide policy-relevant information for mitigation targets, including the 2 degrees C limit goal set by the 2015 Paris Agreement of the United Nations Framework Convention on Climate Change, enabling progress related to multiple United Nations Sustainable Development Goals.