Nitrogen application improves yield threshold and slows leaf nitrogen loss in mid-upper canopy of forage soybean under high planting density

Optimizing planting density and nitrogen application can improve grain crop yield by efficiently allocating dry matter and nutrients. However, the response of forage soybeans, focused on forage yield and quality, to planting density and nitrogen application, particularly in terms of dry matter and nitrogen allocation among organs and canopy layers, remains unclear. Therefore, a three-year field experiment was conducted on the western Loess Plateau to investigate the effects of three planting densities (50,000, 90,000, and 130,000 plants ha −1 ) and nitrogen application rates (0, 40, and 80 kg ha −1 N) on the spatial dynamics of dry matter allocation and nitrogen content in forage soybean plant during the vegetative growth stage. The results indicated that the accumulation and allocation of dry matter and nitrogen content under three plant densities and nitrogen application rates followed similar patterns through growth stages. Increasing the plant density to 130,000 plants ha −1 led to a greater allocation of dry matter to the stems, a rapid decline in leaf nitrogen content, and an increase in yield by 38.7–59.4 %. And nitrogen application further increased the yield threshold due to densification. The combination of 130,000 plants ha −1 with 80 kg ha −1 N was highlighted as it demonstrated optimal agronomic performance, achieving the highest dry matter yield (21.9 t ha −1 ) and crude protein yield (3686.4 kg ha −1 ) among all treatments. Increased density reduced stem diameter, node number, and dry matter partitioning in the mid-lower canopy layer (0–80 cm) and leaf number and nitrogen content in the mid-upper canopy layer (40–120 cm). The dry matter and nitrogen content of canopy 40–80 cm from the bottom responds strongly to density changes. Nitrogen application slowed the nitrogen content loss from densification by increasing leaf volume in the 80–120 cm canopy layer and leaf nitrogen content in the 40–120 cm canopy layer. Our results provide strong evidence on yield formation and nitrogen pattern of forage soybean under plant density and nitrogen rate. These insights are essential for promoting forage soybean planting in the local farming system effectively. • Increased density reduced leaf volume and nitrogen content in mid-upper canopy layer. • High-density increased dry matter production of lower canopy stems and upper canopy leaves. • Nitrogen application improved yield threshold under high planting density. • Nitrogen application slowed the nitrogen content loss under high-density.