Multi-Objective Optimization of Nitrogen Management for Oat Production in the Agro-Pastoral Ecotone of China Using NSGA-II to Balance Yield, Quality, Nitrogen Use Efficiency and Environmental Sustainability

Oat is a staple cereal in semi-arid regions and requires eco-friendly nitrogen management to stabilize yield and mitigate environmental risks. A two-year field experiment was conducted to investigate the impacts of nitrogen rate and topdressing timing on oat grain yield, β-glucan yield, economic return, nitrogen agronomic efficiency, and soil mineral nitrogen residue. Jointing stage topdressing at 105–140 kg·ha−1 markedly raised grain yield and β-glucan yield. Physiological mechanism analysis revealed that appropriate nitrogen topdressing at the jointing stage effectively maintained a high chlorophyll content and nitrogen balance index by significantly improving the activities of nitrate reductase and glutamine synthetase during the grain-filling stage, thereby promoting the accumulation of dry matter and nitrogen. With the increase in nitrogen application rate and the delay of topdressing time, the soil mineral nitrogen accumulation increased significantly. Furthermore, a multi-objective optimization framework integrating regression models with the NSGA-II algorithm identified the optimal strategy as 140.68 kg·ha−1 with topdressing 32 days after emergence. This study constructed a multi-objective optimization system for oat nitrogen application suitable for the agro-pastoral ecotone of northern China, which can provide a theoretical basis and practical methods for the synergistic achievement of high-yield, high-quality oat production and ecological conservation.