Research status and application prospects of combined nitrogen fixation in gramineous plants

Nitrogen is one of the most important factors restricting agricultural production. With the development of artificial nitrogen fixation technology, the application of nitrogen fertilizers can increase crop yields and solve problems related to the fulfilment of the basic human needs of food and clothing. However, it has also caused environmental problems, such as soil compaction, acidification, nitrogen loss, and greenhouse gas emissions (e.g., nitrous oxide, N₂O). Compared with synthetic ammonia, biological nitrogen fixation is a green and economical nitrogen fixation method, which entails symbiotic nitrogen fixation and non-symbiotic nitrogen fixation (autogenous nitrogen fixation and combined nitrogen fixation, respectively). Annually, biologically fixed nitrogen can account for more than 50% of the total fixed amount. Compared with symbiotic nitrogen fixation, non-symbiotic nitrogen fixation exists in many plants, for example, sugarcane, rice, maize, wheat, and other gramineous crops that carry out non-symbiotic nitrogen fixation (combined nitrogen fixation). This article reviewed the species of combined nitrogen-fixing bacteria in gramineous plants and their mechanism of action and nitrogen-fixing activity and regulation methods, as well as the resources and applications of these combined nitrogen-fixing bacteria. Compared with symbiotic nitrogen fixation, combined nitrogen-fixing bacteria are more vulnerable to indigenous microorganisms. Research on combined nitrogen-fixing bacteria is more difficult owing to the influence of environmental factors, such as nitrogen levels. It is necessary to screen and purify more combined nitrogen-fixing bacteria to provide optimum materials for research into the nitrogen fixation mechanism. Appropriate levels of nitrogen, phosphorus, molybdenum, iron, and other fertilizers can promote the nitrogen fixation efficiency of bacteria. Nitrogen-fixing bacteria not only increase the extent of soil nitrogen fixation but also facilitate the regulation of plant root hormones, thereby increasing plant disease resistance and stress resistance, promoting healthier plant growth. Finally, agronomic management measures for combined nitrogen fixation through gramineous plants and the practical application of the nitrogen-fixing bacteria are proposed to provide a theoretical basis for improving the efficiency of combined nitrogen fixation through gramineous plants and to promote the application of the nitrogen-fixing bacteria in agricultural production.