Effects of nitrogen application at different densities on carbon and nitrogen accumulation and translocation characteristics in forage maize in semi-arid regions

To provide a theoretical basis for high-yield and high-quality cultivation of silage maize in semi-arid regions, this study investigated the effects of nitrogen application on carbon and nitrogen accumulation, translocation, and crude protein synthesis in silage maize under different planting densities. Using ‘Longqingzhu 1’ as the experimental material, two planting densities (60,000 and 75,000 plants hm-2) and three nitrogen application regimes (all nitrogen as basal fertilizer, 40% and 60% of nitrogen applied as topdressing at the bell stage) were examined. Measurements included dry matter, non-structural carbohydrate (NSC), and nitrogen accumulation and translocation before and after flowering, as well as their contribution to total plant biomass. Correlations between these physiological parameters and forage yield, crude protein content, and crude protein yield were also analyzed. Compared with CK1 (60,000 plants hm-2 with all nitrogen applied basally), treatments T1 and T2 (same density with 40% and 60% nitrogen topdressing, respectively) showed significantly reduced dry matter and NSC translocation and contribution rates at the pre-flowering stage, but significantly increased post-flowering dry matter accumulation by 46.29% and 22.63%, and NSC accumulation by 29.53% and 17.54%, respectively. Compared with CK2 (75,000 plants hm-2 with all nitrogen applied basally), T3 and T4 (same density with 40% and 60% nitrogen topdressing, respectively) also showed reduced translocation and contribution rates at the pre-flowering stage, while post-flowering dry matter increased by 5.88% and 5.90%, and NSC accumulation increased by 19.53% and 65.57%, respectively. Nitrogen topdressing significantly increased nitrogen translocation and translocation rate at the pre-flowering stage, as well as nitrogen accumulation at the post-flowering stage. Compared with CK1, T1 showed increases of 64.48%, 7.77%, and 74.86%, while T2 increased by 43.21%, 7.52%, and 32.27%, respectively. Compared with CK2, T3 increased by 12.51%, 8.92%, and 74.94%, and T4 by 47.37%, 8.75%, and 73.41%, respectively. Forage yield was significantly positively correlated with post-flowering dry matter accumulation (R2 = 0.1547*) and nitrogen accumulation (R2 = 0.1582*). Crude protein content was positively correlated with pre-flowering nitrogen translocation (R2 = 0.1263*), post-flowering nitrogen accumulation (R2 = 0.1764**), and NSC accumulation (R2 = 0.2155**). The crude protein yield of the whole plant was significantly positively correlated with post-flowering dry matter (R2 = 0.1723*) and nitrogen accumulation (R2 = 0.1682*). These results indicate that nitrogen topdressing significantly promotes crude protein synthesis in silage maize by enhancing nitrogen translocation at the pre-flowering stage and carbon and nitrogen accumulation at the post-flowering stage. The crude protein content of T1 and T2 was 9.42% and 8.40%, which was 62.18% and 44.80% higher than that of CK1, respectively. The crude protein content of T3 and T4 was 7.06% and 8.12%, 39.25% and 60.15% higher than that of CK2, respectively. Therefore, T1 and T4 are recommended for improving silage maize quality in semi-arid regions.