Deep Fertilization Enhances Crude Protein Content in Forage Maize by Modulating Key Enzymes of Protein Synthesis Across Plant Organs in Semi-Arid Regions of China

Appropriate fertilization depth promotes the absorption and transport of nutrients, crop growth and yield. However, little is known about whether deep fertilization improves crude protein synthesis and how to regulate it. A two-year field experiment was conducted with various fertilization depths: (1) conventional fertilization (CF), (2) fertilization application depth at 30 cm (DF), and (3) fertilizer average application at depths of 15 cm and 30 cm (AF). The fertilization rates under all treatments were 300 kg N ha⁻¹ nitrogen fertilizer (urea, 46% N), 150 kg P₂O₅ ha⁻¹ calcium superphosphate (16% P₂O₅), and 135 kg K₂O ha⁻¹ potassium sulfate (51% K₂O). The nitrogen/potassium (N/K) ratio, the activities of nitrate reductase [NR], glutamine synthetase [GS], and glutamic pyruvic transaminase [GPT], crude protein content in leaves, stems, and grains, as well as the relationships among the parameters were explored. The result showed that deep fertilization (DF) significantly improved the N/K ratio. NR activity in DF increased by 26.30%, 35.56%, and 57.30% in leaves, stems, and grains, respectively, when compared to conventional fertilization (CF), and by 54.22%, 43.27%, and 28.44% when compared to average fertilization (AF). GS activity in DF increased by 29.67%, 47.96%, and 47.46% in leaves, stems, and grains when compared to CF, and by 40.05%, 31.51%, and 40.62% when compared to AF. GPT activity in DF was significantly higher than CF and AF in grains, and differences between treatments were significant. Crude protein content was significantly correlated with NR and GS activities in leaves, GPT activity in stems, as well as GS and GPT activities in grains. The crude protein content of leaves and grains in DF was significantly higher than in CF and AF. In conclusion, DF significantly improved crude protein synthesis and increased the crude protein content of forage maize by increasing the whole plant N/K ratio, NR and GS activities in leaves, as well as GS and GPT activities in grains. It is a highly efficient cultivation technology that significantly improves the quality of forage maize.