Improving nitrogen use efficiency of rice crop through an optimized root system and agronomic practices

Nitrogen (N) is the major nutrient limiting rice (Oryza sativa L.) production. Avoiding excessive N use without compromising yields has long been a priority for both research and government policy in China. Nitrogen use efficiency (NUE) reflects the relationship between dry matter production or economic yield and the quantity of N applied. As an important organ for nutrient absorption, the root system of rice is closely related to NUE. We hypothesized that NUE could be improved by optimizing the root system. Here, we described and compared the evaluation methods of NUE in rice and discussed the relationship of rice root morphological (quantity, weight, length, surface area, number, diameter, volume, distribution, and structure) and physiological (root oxidation activity, active absorbing area or rate, hormones, composition and concentration of root bleeding and root exudates, nutrition concentration, and NH₄⁺/NH₃⁻ flux) characteristics with N absorption and utilization. The key agronomic practices were proposed to improve NUE from several aspects, including adoption of N-efficient varieties and improvements in N and water management, farming systems, and soil properties. Further research is needed to clarify the root-soil interface principle based on efficient N uptake, reveal the root‒shoot interaction mechanism based on efficient N utilization, and explore the integrative management strategy of high yield and efficient N utilization through maximizing the root system of rice. Our objective in this review is to provide ideas to further exploit potentialities of the root system through variety selection and cultivation management to improve NUE.