Effects of Plant Density and Row Spacing on Canopy Structure, Light Use Efficiency, and Yield of Drip-Irrigated Soybean

Increasing planting density is a common strategy to raise soybean yield, yet it often intensifies light competition within the canopy, leading to diminishing returns at high densities. Optimizing row spacing to improve canopy light distribution and light use efficiency is therefore key to increasing yield under dense planting. This study examined the combined effects of planting density and row spacing on canopy light interception, distribution dynamics, and yield in a drip-irrigated soybean system. A two-year field experiment (2024–2025) was conducted in Shihezi, Xinjiang, using three density levels (D1: 210,000; D2: 330,000; D3: 450,000 plants ha−1) and two row spacing patterns (RS1: alternating wide–narrow rows of 20 + 55 cm; RS2: uniform 38 cm rows). Results demonstrated that plant density establishes the baseline for yield, while row spacing modulates light utilization and unlocks yield potential under high-density conditions. The RS1 treatment increased SPAD values in upper leaves by 6.06% at the R6 growth stage compared to the RS2 treatment. At the R5 stage, the RS1 treatment increased radiation use efficiency (RUE) by an average of 6.44%. This planting pattern alleviated photosynthetic decline in dense canopies and conferred a distinct yield advantage. The highest yield was achieved under the D2 treatment, which was 8.44% and 6.71% higher than that under the D1 and D3 treatments, respectively. In conclusion, integrating moderate plant density with optimized wide–narrow row spacing improves canopy light capture and utilization, synergistically enhancing yield and resource use efficiency. This approach offers a practical strategy to overcome the yield plateau in high-density soybean production systems.