Optimization of water and fertilizer coupling strategies for walnut based on yield, quality, and water fertilizer use efficiency

【Objective】Walnut (Juglans regia L.) is a crucial tree species with both ecological and economic importance. In China, it plays a significant role in agricultural development, particularly in the southern region of Xinjiang, where walnut cultivation constitutes a major source of income for local farmers. As of 2024, the total walnut cultivation area in Xinjiang reached 6.4 million mu, with an output of approximately 1.445 million tons, accounting for about 22% of the national production. The total output value exceeded 17 billion CNY, demonstrating a significant year-on-year increase. However, the arid climatic conditions, sandy soil texture, and high evaporation rates in the region pose challenges tosustainable walnut production. Traditional surface irrigation methods often result in low water use efficiency and accelerate nutrient leaching, further exacerbating resource waste and limiting yield potential. To address these challenges, this study investigated the effects of different irrigation and fertilization levels under micro- sprinkler fertigation conditions on walnut yield, nut quality, and resource use efficiency. The aim was to determine the optimal water and fertilizer input strategy during the walnut growing season to enhance yield and quality while improving water and nutrient use efficiency. This approach is expected to provide a sustainable solution to water and nutrient management, ultimately achieving water-saving, fertilizer-saving, and yield-increasing goals in arid regions.【Methods】A field trial was conducted using the walnut cultivar Wen 185 as the experimental material. A split-plot design was adopted with two irrigation levels and three fertilization levels, resulting in six water-fertilizer coupling treatments in total. An additional treatment following local farmers' conventional management practices was included as a control (CK). The experimental setup aimed to explore the interactive effects of irrigation and fertilization levels on walnut fruit quality, yield, water use efficiency (WUE), and partial factor productivity of fertilizer (PFP). Key parameters measured included fruit biochemical composition (e.g., fat content, cellulose, reducing sugars, total sugars, soluble protein, tannins, and total phenolics), yield per unit area, and efficiency indicators. To further identify the optimal combination of irrigation and fertilization, a dual-variable second-order polynomial regression model was constructed for yield, WUE, and PFP. Multi- objective optimization was performed using the Non- dominated Sorting Genetic Algorithm Ⅱ (NSGA-Ⅱ), which efficiently searched the solution space for the optimal fertigation strategy.【Results】Walnut yield, WUE, and PFP exhibited a non-linear response to increasing water and fertilizer inputs, generally showing a trend of initial increase followed by decline. This indicated that excessive input may lead to diminishing returns or even adverse effects. Compared with irrigation, fertilization had a greater influence on yield and WUE, though a significant synergistic interaction between water and fertilizer was observed. Under fixed irrigation conditions, both WUE and PFP increased with higher fertilization up to a certain point, beyond which they began to decline. A similar trend was observed when fertilization level was held constant and irrigation amount varied. The best performance in terms of yield, quality, and resource efficiency occurred under moderate irrigation and fertilization levels, demonstrating the importance of balanced inputs. Nut quality was significantly influenced by different water-fertilizer coupling treatments. Specific combinations of irrigation and fertilization markedly enhanced the accumulation of bioactive compounds (e.g., total phenolics), cell wall components (e.g., cellulose), sugars, and fats in walnut kernels. Both excessive (high water and high fertilizer) and deficient (low water and low fertilizer) treatments suppressed walnut growth and development. In contrast, an appropriate water- fertilizer combination significantly promoted nut quality and yield. Compared to the low water-low fertilizer treatment, the optimized fertigation strategy increased irrigation by approximately 91-98 m3 ·666.7 m-2 and fertilization by 52-63 kg·666.7 m-2 . This resulted in increases in reducing sugar (by 0.27% ), total sugar (by 0.02% ), yield (by 135-136 kg · 666.7 m- 2 ), and WUE (by 0.04-0.08 kg·mm-1 ·666.7 m-2 ), while fat content, cellulose, soluble protein, tannins, total phenolics, and PFP decreased by 5.25%, 0.34%, 0.14 mg·g-1 , 0.55 mg·g-1 , 2.31 mg·g-1 , and 3.65-6.71 kg·kg-1 , respectively. When compared to the local farmers’conventional practice (CK), the optimized treatment increased fat content by 0.08% , reducing sugar content by 0.35% , yield by 145- 149 kg · 666.7 m- 2 , WUE by 0.29-0.34 kg·mm-1 ·666.7 m-2 , and PFP by 14.86-17.91 kg·kg-1 . Meanwhile, cellulose, soluble protein, tannins, total sugar, and total phenolics decreased by 0.48%, 0.23 mg · g-1 , 1.52 mg · g-1 , 0.03%, and 1.09%, respectively. These results indicated that optimized fertigation can lead to substantial im-provements in both yield and resource use efficiency, with slight trade-offs in some quality traits.【Conclusion】Irrigation and fertilization levels significantly influence walnut fruit quality and yield, with a clear synergistic effect observed. The relationships between yield, WUE, and PFP with water and fertilizer input levels followed a parabolic trend, suggesting that moderate input levels are critical to maximizing production efficiency. Based on NSGA-Ⅱ optimization results, the optimal fertigation strategy for walnut during the growing season was determined to be an irrigation volume of 498.12 m3 ·666.7 m-2 and a fertilization amount of 139.72 kg·666.7 m-2 . Under this regime, fat content reached 59.08%, yield reached 410.73 kg · 666.7 m-2 , WUE was 0.75 kg ·m-3 , and PFP reached 44.44 kg ·m-3 . Furthermore, dynamic monitoring of both internal and external walnut fruit quality indicators throughout the growing season demonstrated close agreement with the model-predicted optimal irrigation levels, confirming the accuracy and practical relevance of the optimized fertigation strategy. The high consistency between empirical monitoring and model- based simulations underscores the reliability of this approach and provides a scientific basis for water and nutrient management in walnut production under arid conditions. These findings offer valuable guidance for promoting resource-efficient and sustainable walnut cultivation in Xinjiang and similar agroecological regions.