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High fertilizer application and over-irrigation in sugarcane systems can cause considerable N₂O emissions. Optimized water-fertilization management which reduces N₂O emissions, while maintaining sugarcane biomass, is crucial, but may affect nitrogen fixation by sugarcane. This study evaluated the combined effect of water-fertilization on sugarcane biomass and nitrogen fixation in field trials in southern China. Treatments included drip and spray irrigation, with three levels (0% (low), 50% (medium), 100% (high)) of irrigation and of fertilizer. A rain-fed crop (no irrigation or fertilizer) was included as the control. The results showed that (1) spray irrigation with medium water and high fertilization increased biomass. The optimum combination in sugarcane elongation stage was drip irrigation with medium water and high fertilization, while drip irrigation with high water and high fertilization was the best choice for maturity stage. (2) For sugarcane nitrogen (δ¹⁵N) content, spray irrigation with medium water and high fertilization was the best combination in seedling and tillering stages. The optimum combination in the elongation stage was drip irrigation with medium water and high fertilization, and in maturity stage was drip irrigation with high water and high fertilization. (3) For soil (δ¹⁵N content), drip irrigation with high water and high fertilization proved optimal for seedling, tillering, and maturity stages. (4) In seedling stage, sugarcane (δ¹⁵N content) was found to be strongly correlated with leaf area index, soil water, soil temperature, and soil electrical conductivity. Soil (δ¹⁵N content) was correlated with photosynthesis and soil temperature. In conclusion, drip irrigation appears most suitable for field planting, while the best treatment in seedling and tillering stages is medium water-high fertilization, and that the best in elongation stage is high water-medium fertilization. The optimum water-fertilizer combinations identified here can provide a scientific basis for optimization and management of irrigation and fertilization in China and other regions with similar environments.