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

Abstract

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 NH 3 , NO x , and N 2 O. 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 N 2 O emissions dominate over local cooling effects ascribed to its NH 3 and NO x 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°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.

Data‐driven estimates of fertilizer‐induced soil NH3, NO and N2O emissions from croplands in China and their climate change impacts

“…Hence, more long‐term Nr field measurements and the development of process‐based ecosystem models or robust bottom‐up approaches are expected to further constrain such uncertainties. To address the limitation, we have refined the direct EFs of Nr in our estimates based on direct measurement data by taking into consideration their variation among different N fertilizer types, regions and cropping systems, instead of the use of coarse national statistical data and IPCC default or uniform EFs as reviewed in previous studies (de Klein et al, 2006; Wang et al, 2020; Xu et al, 2020; Zou et al, 2010). Furthermore, a complete accounting of croplands Nr (NH 3 , NO and N 2 O) emissions in China from the use of N fertilizer has been rarely addressed, especially by the collection of direct measurement data.…”

“…Uncertainties, however, still existed for current estimates of studies (de Klein et al, 2006;Wang et al, 2020;Xu et al, 2020;Zou et al, 2010). Furthermore, a complete accounting of croplands…”

“…Recently, several studies have attempted to quantify N fertilizer‐induced soil Nr emissions from croplands in China, but large uncertainties still existed for these estimates, generally showing a range from 1.89 to 8.09 Tg N yr −1 for NH 3 , from 0.11 to 0.27 Tg N yr −1 for NO and from 0.19 to 0.45 Tg N yr −1 for N 2 O (Gerber et al, 2016; Gu et al, 2012; Xu et al, 2020; Zhou et al, 2014). These uncertainties are primarily attributed to diverse estimation methods and differences in agricultural activity data.…”

“…The extensive use of N fertilizer in crop production also incurs substantial soil Nr emissions to the atmosphere. The total Nr emissions from global croplands to the atmosphere have risen from 14 to 35 Tg N yr −1 between 1970 and 2012, representing roughly 31% of the global yield of total Nr emissions today (Xu et al, 2020). In addition, this nitrogen flux is projected to double by 2050 to keep pace with the increase in global food demand (Bodirsky et al, 2014).…”

The Fate of Nitrogen from Soil to Plants: Influence of Agricultural Practices in Modern Agriculture

“…An excessive quantity of nitrogen fertilizers is supplied in the soil, because only about half of the applied quantity is available and is finally absorbed by crops [6]. The other half is dispersed in the environment in different forms [5,7]. Staple foods such as Brassica napus L., used for oil production in China are cultivated by using about 300 kg N ha −1 , but actually plants take up only 30-50% of the total nitrogen supplied [8].…”