Optimizing nitrogen and maize plant density to improve competitive advantage and fertilizer use efficiency in maize soybean intercropping in Northwest China

Maize-soybean intercropping is a critical ecological planting technique that embodies the essence of China's intercropping agricultural practices, enhancing farmland productivity while preserving soil health and ensuring sustainable production. However, the interplay between nitrogen management and planting density, along with interspecific interaction/competition dynamics in maize-soybean strip intercropping under rain-fed conditions, remains unclear; elucidating these factors is critical for optimizing resource utilization efficiency in arid farming systems. This study employed a split-plot field design under maize-soybean intercropping conditions, with maize planting density as the main plot factor (52500, 60000 and 67500 plants ha⁻¹) and nitrogen application rate as the subplot factor (300, 375 and 450 kg ha⁻¹). Maize and soybean monocultures were included as controls. Results demonstrated that increasing planting density and nitrogen application significantly affected crop growth, photosynthetic characteristics, yield, and economic return in the maize-soybean intercropping system. With increasing planting density from D1 to D3, maize yield increased first and then declined, whereas soybean yield decreased overall. Increasing planting density altered maize growth traits, suppressed soybean growth, decreased leaf vapor pressure deficit(VPD), and generally increased net photosynthetic rate (Pn). With increasing nitrogen application from N1 to N3, crop growth and yield formation were improved under D2, but under D3, excessive nitrogen input reduced transpiration and weakened productivity. Compared with D1, maize yield under D2 increased by 28.76%, whereas soybean yield under D3 decreased by 36.15%. Therefore, N2D2 (60,000 plants ha⁻¹ combined with 375 kg N ha⁻¹) was identified as the optimal planting strategy for maize-soybean intercropping under rain-fed conditions in southern Ningxia.