The demand for nitrogen (N) for crop production increased rapidly from the middle of the twentieth century and is predicted to at least double by 2050 to satisfy the on-going improvements in productivity of major food crops such as wheat, rice and maize that underpin the staple diet of most of the world’s population. The increased demand will need to be fulfilled by the two main sources of N supply – biological nitrogen (gas) (N₂) fixation (BNF) and fertilizer N supplied through the Haber-Bosch processes. BNF provides many functional benefits for agroecosystems. It is a vital mechanism for replenishing the reservoirs of soil organic N and improving the availability of soil N to support crop growth while also assisting in efforts to lower negative environmental externalities than fertilizer N. In cereal-based cropping systems, legumes in symbiosis with rhizobia contribute the largest BNF input; however, diazotrophs involved in non-symbiotic associations with plants or present as free-living N₂-fixers are ubiquitous and also provide an additional source of fixed N. This review presents the current knowledge of BNF by free-living, non-symbiotic and symbiotic diazotrophs in the global N cycle, examines global and regional estimates of contributions of BNF, and discusses possible strategies to enhance BNF for the prospective benefit of cereal N nutrition. We conclude by considering the challenges of introducing in planta BNF into cereals and reflect on the potential for BNF in both conventional and alternative crop management systems to encourage the ecological intensification of cereal and legume production.

自二十世纪中叶起，作物生产对氮 (N) 的需求迅速增加，预计到 2050 年将至少增加一倍，以满足支撑小麦、水稻和玉米等主要粮食作物生产力不断提高的需求世界上大多数人的主食。增加的需求需要通过两个主要的氮供应来源来满足——生物固氮（气体）（N ₂）固定（BNF）和通过哈伯-博世工艺提供的肥料氮。BNF 为农业生态系统提供了许多功能优势。它是补充土壤有机氮库和提高土壤氮的可用性以支持作物生长的重要机制，同时还有助于降低肥料氮的负环境外部性。在以谷物为主的种植系统中，豆类与根瘤菌共生贡献最大的BNF输入；然而，参与与植物非共生关系或作为自由生活的 N ₂固定剂存在的固氮菌是普遍存在的，并且还提供了固定 N 的额外来源。这篇综述通过自由生活、非共生和全球氮循环中的共生固氮菌，研究了全球和区域对 BNF 贡献的估计，并讨论了增强 BNF 以实现谷物氮营养的预期效益的可能策略。最后，我们考虑了将植物BNF引入谷物中的挑战，并反思了 BNF 在传统和替代作物管理系统中的潜力，以鼓励谷物和豆类生产的生态集约化。