Performance of combined irrigation–fertilization strategies under variable inflow and cutoff conditions in border irrigation systems

Efficient irrigation–fertilization management is crucial for sustaining high crop productivity while minimizing nutrient losses and environmental risks. This study examined the interactive effects of inflow rate, cutoff ratio (CR), and fertilizer application length (F) on nitrogen distribution and retention under border irrigation. Field experiments were conducted in 100 m long border-irrigated wheat fields with an average longitudinal slope of 0.0023 m/m, using high (6.91 L m⁻¹ s⁻¹), moderate (4.95 L m⁻¹ s⁻¹), and low (2.81 L m⁻¹ s⁻¹) inflow rates, combined with variable CR and F levels (representing the proportion of border length receiving fertilizer). Results showed that nitrogen distribution and retention were strongly influenced by the interaction between CR and F, and the optimal strategy depended on inflow rate. Under high inflow conditions, the F80 %–CR0.90 combination achieved the most uniform nitrogen distribution within the root zone, with minimal downward movement. For moderate inflow, F80 %–CR0.80 produced a balanced horizontal and vertical nitrogen profile, maintaining high nitrogen use efficiency with low deep percolation. Under low inflow conditions, the F100 %–CR1.00 strategy was most effective, ensuring uniform nutrient distribution, maximizing retention within the 0–60 cm root zone, and minimizing leaching losses. Shorter fertilization lengths (F60 %) led to nitrogen accumulation near the inflow zone and downstream depletion, while inappropriate cutoff ratios either promoted leaching or restricted fertilizer transport. Overall, the results demonstrate that optimal irrigation–fertilization strategies are inflow-dependent, with F80 % being optimal for high and moderate inflow rates, and F100 %–CR1.00 performing best under low inflow. The proposed approach provides a practical framework for improving water–nutrient coordination in border irrigation, enhancing nitrogen use efficiency, and promoting sustainable wheat production in water-limited agricultural regions.