Sustainable quinoa production in semi-arid regions is constrained by water scarcity and inefficient photoassimilate partitioning. This study evaluated the interactive effects of planting patterns and superabsorbent polymer (SAP) application rates (0, 60, and 90 kg·ha⁻¹) on ‘Longli 5’ quinoa source–sink dynamics. Results demonstrated that ridge-furrow planting coupled with a moderate SAP rate (60 kg·ha⁻¹, D1) synergistically optimized the soil moisture environment, enhancing the two-year average leaf area index (LAI) by 52.6–66.9% and net photosynthetic rate (Pn) by 5.45–10.64% (p < 0.05). Unlike the high SAP rate (90 kg·ha⁻¹), “water luxury consumption” and low input efficiency dynamic which induced “water luxury consumption” and low input efficiency, the D1 treatment effectively promoted photoassimilate remobilization to grains by 13.39–14.44%. Unlike high SAP rates (90 kg·ha⁻¹), which induced ‘water luxury consumption’ and low input efficiency, the D1 treatment effectively promoted photoassimilate remobilization to grains by 13.39–14.44%.This mechanism optimized source–sink coordination, resulting in a significant increase in seed yield and thousand-grain weight (p < 0.05). These findings conclude that a balanced source–sink allocation, rather than maximal source capacity alone, is the definitive driver of productivity. Integrating ridge-furrow planting with moderate SAP application (60 kg·ha⁻¹) is a highly efficient agronomic strategy for harmonizing quinoa growth and achieving sustainable, high yields in semi-arid environments.

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.