Combining nitrogen effects and metabolomics to reveal the response mechanisms to nitrogen stress and the potential for nitrogen reduction in maize

The physiological and metabolic differences in maize under different nitrogen (N) levels are the basis of reasonable N management, which is vital in improving fertilizer utilization and reducing environmental pollution. In this paper, on the premise of defining the N fertilizer efficiency and yield under different long-term N fertilization treatments, the corresponding differential metabolites and their metabolic pathways were analyzed by untargeted metabolomics in maize. N stress, including deficiency and excess, affects the balance of carbon (C) metabolism and N metabolism by regulating C metabolites (sugar alcohols and tricarboxylic acid (TCA) cycle intermediates) and N metabolites (various amino acids and their derivatives). L-alanine, L-phenylalanine, L-histidine, and L-glutamine decreased under N deficiency, and L-valine, proline, and L-histidine increased under N excess. In addition to sugar alcohols and the above amino acids in C and N metabolism, differential secondary metabolites, flavonoids (e.g., kaempferol, luteolin, rutin, and diosmetin), and hormones (e.g., indoleacetic acid, trans-zeatin, and jasmonic acid) were initially considered as indicators for N stress diagnosis under this experimental conditions. This study also indicated that the leaf metabolic levels of N2 (120 kg ha–1 N) and N3 (180 kg ha–1 N) were similar, consistent with the differences in their physiological indexes and yields over 12 years. This study verified the feasibility of reducing N fertilization from 180 kg ha–1 (locally recommended) to 120 kg ha–1 at the metabolic level, which provided a mechanistic basis for reducing N fertilization without reducing yield, further improving the N utilization rate and protecting the ecological environment.