Mitigating salt stress in brackish water-irrigated cotton fields via nitrogen input: Unveiling drivers and pathways through multi-objective optimization

Brackish water irrigation is crucial for water-scarce arid regions, yet its long-term use often leads to harmful soil salt accumulation. This salinization threatens soil health, disrupts nitrogen transformation, reduces nitrogen availability, and ultimately impairs cotton's nitrogen use efficiency. Optimizing nitrogen management under such conditions is therefore essential for sustaining productivity. To address this issue, a three-year consecutive field experiment was conducted. In the 2022–2023 season, three irrigation salinity treatments were established: 1 (S1), 3 (S2), and 5 (S3) g L −1 . In 2024, nitrogen application treatments were incorporated, consisting of 200 (N1), 300 (N2), and 400 (N3) kg hm −2 , enabling the assessment of combined salt-nitrogen interactions on soil processes and cotton performance. The study examined how brackish water salinity and nitrogen application rate jointly influence soil properties and yield. The results indicated that the S2 treatment yielded the highest. However, three years of irrigation increased soil salinity and moisture under S2 while decreasing nutrients. The combination of N2S2 significantly increased seed cotton yield by 17.69 %-19.59 % and water use efficiency by 11.49 %-34.28 %, while maintaining lower salt accumulation. Random Forest analysis identified soil salinity as the dominant factor controlling yield and fiber quality, while soil nitrate and ammonium nitrogen were the key predictors for nitrogen use efficiency. Partial Least Squares Path Modelling revealed that brackish water and nitrogen fertilizer can improve the soil environment by regulating water-salt dynamics and nutrient status, thereby enhancing overall crop performance. Based on multi-objective optimization using the Non-dominated Sorting Genetic Algorithm-III and decision-making with CRITIC-TOPSIS, the recommended management combination for the study area is a nitrogen application rate of 281.03 kg hm −2 and an irrigation water salinity of 2.34 g L −1 . This strategy effectively balances productivity, fiber quality, and resource-use efficiency, providing a practical pathway for sustainable cotton production and soil health preservation in arid regions under brackish water irrigation. • Brackish water (>3 g L −1 ) irrigation raises soil salinity. • Soil salinity is the dominant factor affecting cotton yield. • Brackish water and nitrogen enhances soil by balancing water-salt and nutrients. • A nitrogen rate of 281 kg hm −2 with 2.34 g L −1 irrigation salinity is recommended.