Irrigation water is increasingly recognized as a potential nutrient source in intensive vegetable systems, particularly in peri-urban areas influenced by organic and wastewater inputs. This study evaluated the effects of organic-rich irrigation water on yield and quality of mustard greens (Brassica juncea L.) grown on Red River alluvial soil, and quantified its contribution to nitrogen supply. A greenhouse experiment using a randomized complete block design with three replications and four treatments compared Cau Bay canal water, treated domestic wastewater, and groundwater, with and without chemical fertilization, across three cropping seasons. Organic-rich irrigation significantly improved plant growth and yield compared with unfertilized groundwater, supplying approximately 19-22 kg N ha-1 per crop season (27.5-31.0% of the recommended nitrogen rate). Yield increased by 30-70%, although maximum productivity still depended on mineral fertilization. Nitrate accumulation rose consistently with nitrogen availability and was strongly associated with yield and root traits, indicating that nitrogen supply simultaneously enhances productivity and nitrate storage. Multivariate analysis showed that growth, yield, and nitrate accumulation aligned along a common gradient governed by nitrogen availability, revealing a system-level trade-off between productivity and product quality. These results demonstrated that irrigation water functions not only as a water source but also as an active nutrient carrier influencing crop performance and nutrient dynamics. Integrating irrigation water quality into fertilization strategies is therefore essential to improve resource use efficiency while mitigating risks of nitrate accumulation, food safety concerns, and environmental impacts.