Objectives Coordination of N supply and plant density is essential for high soybean yield. Here, we studied the effect of N fertilizer combination and plant density on the photosynthetic performance and yield of summer soybean.

Methods Field experiments were conducted in the Agronomy Experiment Station of Shandong Agricultural University from 2018—2020. Summer soybean cultivar Qihuang 34 (QH34) was used as experimental materials. We tested four planting densities: 90000 (D1), 120000 (D2), 150000 (D3) and 180000 plants/hm² (D4). The fertilizer treatments were urea (U), chicken manure (M), and 50% urea and 50% chicken manure (UM), and no nitrogen fertilizer as control (N0). We analyzed N content, gas exchange, the photosynthetic performance of new fully expanded leaves, and soybean yield.

Results Application of N fertilizer significantly increased soybean yield. M and UM had significantly higher yield than U under all the four density treatments; UM yield was significantly increased than M in 2018 under all the four densities but not significant under high-density treatment (D4) in 2019. There was no significantly difference in the yield of D2, D3 and D4 in 2020. D2 or D3 was more suitable for soybean production in the Huanghuaihai region than D1 and D4. Application of N fertilizer significantly increased the net photosynthetic rate (P_n), light absorption (φ_Po, PI_ABS), energy transformation (φ_Eo), electron transfer activities (Ψ_o), the activities of PSⅡ acceptor side (V_j) and donor side (W_k), and the contribution of N units to PS Ⅱ ability (φ_Po/SLN, W_k/SLN, V_j/SLN, φ_Eo/SLN, Ψ_o/SLN). Also, we found that N fertilization increased the positive correlation between P_n and PSⅡ and P_n/SLN and PSⅡ/SLN. The photosynthetic efficiency of soybean was in UM > M > U in 2018, but there was no significant difference between M and UM in 2020. Under the same N fertilizer application, however, the photosynthetic efficiency of D2 and D3 treatments were similar. The P_n was significantly and positively correlated with PSⅡ, P_n/SLN and the contribution of N units to PS Ⅱ ability. Therefore, P_n increase was mainly due to the improvement in PSⅡ performance after nitrogen fertilizer application, which ultimately affected yields.

Conclusions In the Huanghuaihai region, the optimal plant density of summer soybean is 150000 plants/hm² (D3). Without a change in the N input rate, urea and chicken manure (UM) combination shows a more satisfactory effect on the photosynthetic efficiency and soybean yield than urea or chicken manure alone, especially in the first and second year. However, chicken manure shows a similar effect since the third year. Therefore, the combination of stable organic fertilizer input and a medium-high planting density is more conducive to high yield, efficiency, and quality soybean production in this area.