Abstract:

Intercropping has been widely recognized for promoting efficient resource utilization and increasing productivity; however, the coordinated effects of water-nitrogen regulation on canopy light use, carbon-nitrogen assimilation, grain filling, and system productivity in maize/soybean intercropping remain unclear. A two-year (2023–2024) maize/soybean intercropping field experiment was conducted under different irrigation levels (60%, 80%, and 100% ETc, ETc is crop evapotranspiration) and nitrogen levels (120, 180, 240, and 300 kg N ha⁻¹ for maize). The study revealed that increasing irrigation and nitrogen levels lowered inter-row temperature, increased humidity, and enhanced canopy shading, resulting in a U-shaped vertical distribution of photosynthetically active radiation. The extended row crop radiation transmission model indicated that light interception dynamics varied between maize and soybean across growth stages, with maize peaking at 0.81–0.92 during tasseling, while soybean reached a minimum of 0.14–0.35 at the beginning pod. Canopy light use efficiency varied with species and growth stage, ranging from 0.94 to 2.90 g MJ⁻¹ in maize and 0.95–4.06 g MJ⁻¹ in soybean, with both crops maintaining relatively high efficiency under irrigation of 80% ETc and nitrogen of 240 kg N ha⁻¹ treatments. In addition, compared with other treatments, irrigation of 80% or 100% ETc and nitrogen of 240 kg N ha⁻¹ treatments maintained the highest leaf carbon-nitrogen metabolic capacity, increasing maize ear leaf area and nitrogen content, reducing the leaf carbon-nitrogen ratio, and enhancing the activities of carbon and nitrogen assimilation enzymes (Rubisco, PEPC, NR, GS, and GOGAT) by 14–85%. Consistently, irrigation of 80% ETc and nitrogen of 240 kg N ha⁻¹ treatments improved the maize grain filling process, resulting in the highest maize yield and net incomes of 30162 CNY ha⁻¹ in 2023 and 28692 CNY ha⁻¹ in 2024. Correlation analysis indicated that light interception was positively influenced with carbon-nitrogen assimilation and light use efficiency, thereby enhancing the grain filling rate, maize yield, and economic income. Optimization based on a bivariate regression model identified an optimal irrigation range of 382–388 mm and a nitrogen range of 220–251 kg N ha⁻¹, achieving high productivity in maize/soybean intercropping in northwest China.