Response of canopy light, nitrogen distribution, and nitrogen accumulation characteristics in densely planted soybean to uniconazole treatment

To enhance soybean yield and clarify the role of uniconazole (UZ) in dense planting systems, a field experiment was conducted in 2022−2023 with five planting densities (15×10⁴, 18×10⁴, 21×10⁴, 24×10⁴, and 27×10⁴ plants∙hm⁻²) and two UZ treatments (60 mg∙L⁻¹ UZ and fresh water control). The effects of UZ on canopy light and nitrogen distribution, nitrogen accumulation, and allocation characteristics were examined under varying planting densities. As planting density increased, the fraction of PAR intercepted by each canopy layer (FIPAR) in the upper canopy increased, whereas it decreased in the middle and lower canopy layers. Concurrently, nitrogen content of each canopy layer (NCC) in the upper canopy initially increased and then decreased, whereas NCC in the middle and lower canopy layers decreased. UZ application reduced FIPAR in the upper canopy but increased it in the middle canopy, with a minimal effect on the lower canopy. Moreover, UZ application significantly increased NCC, with the largest effect observed in the middle canopy. As planting density increased, both the light extinction coefficient (K_L) and nitrogen reduction coefficient (K_b) increased, whereas UZ application reduced both. Without UZ application, the matching coefficient of light and nitrogen (K_L/K_b) decreased as the planting density increased, whereas UZ application narrowed the gap between K_L/K_b and the ideal value. Temporally, as the planting density increased, the maximum nitrogen accumulation (Y_m) and its maximum rate of accumulation (V_m) initially increased and then decreased. The time reached the maximum rate of accumulation (t_m) occurred earlier and the duration of rapid accumulation (T) shortened (excluding D18U60). UZ application delayed t_m, extended T, and increased both Y_m and V_m. K_L and K_b were negatively correlated with K_L/K_b and the eigenvalues associated with the rapid nitrogen accumulation phase and yield were positively correlated with Y_m, starting date of rapid accumulation period (t₁), terminating date of rapid accumulation period (t₂), V_m, t_m, and T. Increasing planting density promoted biomass and nitrogen accumulation in vegetative organs, but reduced the economic coefficients for biomass and nitrogen, as well as the nitrogen content in each organ. UZ application increased biomass and nitrogen accumulation in reproductive organs, as well as the economic coefficients for biomass and nitrogen. Consequently, the highest soybean yields were achieved at planting densities of 18×10⁴ plants∙hm⁻² under fresh water treatment and 21×10⁴ plants∙hm⁻² under UZ treatment. In conclusion, the spatial distribution of canopy light and nitrogen influenced the temporal dynamics of nitrogen accumulation, distribution, and yield. UZ application optimized nitrogen accumulation characteristics by coordinating canopy light and nitrogen distribution, increased the optimal planting density from 18×10⁴ to 21×10⁴ plants∙hm⁻², and raised yield by 11.9%–23.2%.