Reactive nitrogen (N) impacts all UN sustainable development goals. Nitrogenous fertilizers have enhanced food security and without them, half of our current human population would not be alive. However, reactive N pollution has crossed earth's planetary boundary of sustainability (Steffen et al., 2015), and nitrogenous fertilizers are among its major contributors. The loss of nitrous oxide, ammonia, nitrates, etc. from unused N-fertilizers causes adverse impacts on soil, water, air, climate change, and biodiversity loss. Billions of dollars per annum are also spent on N-fertilizers globally (Sutton et al., 2019). For over two decades, the International Nitrogen Initiative (INI) has been galvanizing scientists and policy makers to overcome this growing challenge from agriculture, fossil fuel burning, and other anthropogenic sources (www.initrogen.org). Its efforts culminated in the recent intergovernmental recognition of this global concern and the adoption of the first ever India-led UN resolution on sustainable nitrogen management (Raghuram and Sharma, 2019; UNEP, 2019; Raghuram et al., 2021; Sutton et al., 2021).
The resolution heralds the global transition from highlighting problems to finding solutions. Some regions/countries of the world have already quantified their N-losses, such as Europe (Sutton et al., 2011), India (Abrol et al., 2017), and Pakistan (Aziz et al., 2021), while others will do their own assessments for informed policy interventions. The solutions for crop N use efficiency (NUE) have to come from the integration of plant biology, agronomy, genetics, and allied disciplines, as there is no single means to improve crop NUE. Open access journals like Frontiers can accelerate this process with frequent calls for submissions on this topic. This collection of research and review articles is not only very timely from this perspective, but is also becoming a series, considering that another call is already open (https://www.frontiersin.org/research-topics/25179/nitrogen-use-efficiency-plant-biology-to-crop-improvement). The uniquely interdisciplinary nature of Frontiers journals and their collaboration on Research Topics complements other subject-wise collections, such as this triennial on plant NUE (https://academic.oup.com/jxb/issue/71/15) and this INI-led focus on environmental aspects (https://iopscience.iop.org/journal/1748-9326/page/Focus_on_Reactive_Nitrogen).