Nitrogen (N) is an essential constituent of all living organisms and a primary nutrient limiting life on Earth, mediating critical biogeochemical processes across the atmosphere, hydrosphere, biosphere, and pedosphere. The natural nitrogen cycle is a finely tuned network of microbial, physical, and chemical transformations that regulate the availability of reactive nitrogen (Nr) while maintaining environmental equilibrium. However, anthropogenic activities—particularly industrial nitrogen fixation via the Haber–Bosch process, intensive agriculture, and fossil fuel combustion—have drastically perturbed this cycle, leading to a tripling of global Nr inputs since the pre-industrial era. This perturbation has triggered cascading environmental crises, including eutrophication of aquatic ecosystems, atmospheric pollution, greenhouse gas emissions, and biodiversity loss, while simultaneously threatening food security and human health. Achieving a sustainable nitrogen cycle is therefore central to addressing global sustainability challenges, including the United Nations Sustainable Development Goals (SDGs). In this Review, we synthesize the latest advances in our understanding of the natural nitrogen cycle and its anthropogenic perturbations, highlight the complex interactions between nitrogen cycling and climate change, food-energy-water (FEW) nexus, and ecosystem resilience, and outline actionable pathways to restore and sustain nitrogen balance at local, regional, and global scales. We emphasize the need for integrated, transdisciplinary approaches that combine microbial ecology, biogeochemistry, agronomy, policy, and technology to mitigate Nr losses, enhance nitrogen use efficiency (NUE), and reconcile food production with environmental protection. Finally, we identify key knowledge gaps and future research priorities that will be critical for advancing toward a sustainable nitrogen cycle and safeguarding planetary health.
Fanghua Li (Thu,) studied this question.