Objectives Nitrate (NO₃⁻-N) and ammonium (NH₄⁺-N) are the two main forms of inorganic nitrogen (N) absorbed by plants from soil. Maize (Zea mays L.) is a typical dry-land crop, some studies proved that NO₃⁻-N is the major form for maize absorption, however, there are a fewer of studies showing that supplying NH₄⁺-N was more beneficial for the growth of maize than supplying NO₃⁻-N. In this paper, we summarized the N source preference of maize in different environments, and the gene families related to the absorption and transportation of NO₃⁻-N or NH₄⁺-N in maize, aiming to clarify the response of physiological processes such as root development, carbon and nitrogen metabolism, photosynthesis, mineral element absorption, and stress resistance to the two N forms, in order to provide a basis for efficient N utilization and optimized N fertilizer management in maize.

Major advances The N source preference of maize varied depending on the external environmental factors, among which the medium pH was considered as one of important factors. Maize grown in acidic soils preferred NO₃⁻-N, while in alkaline soils it preferred NH₄⁺-N. Compared to the single N form, appropriate NO₃⁻-N/NH₄⁺-N ratios were more conducive to N absorption and yield improvement in maize. In the maize genome, 4 NRT2 is responsible for encoding high affinity nitrate transport system, 79 NPF for low affinity nitrate transport system, 8 AMT for high affinity ammonium transport system, and 2 AMF1 for low affinity ammonium transport system. Nitrate and NH₄⁺-N not only serve as important nutrient resources, but also as signaling factors that participate in and regulate maize root development, carbon and nitrogen metabolism, photosynthesis, mineral element absorption, and stress resistance.

Expectations In order to improve the N-fertilizer utilization efficiency of maize and clarify the mechanism of N forms affecting maize growth, development, and physiological processes, further researches should focus on the molecular mechanisms of absorbing and transporting NO₃⁻-N and NH₄⁺-N by maize, the mechanism of promoting maize growth by the mixed application of NH₄⁺-N and NO₃⁻-N, and field validation experiments on the impact of N form on maize growth and yield.