Coordinated Water–Nitrogen Management for Sustainable Fragrant Pear Production in Arid Regions: Organ Nutrition Regulation and ¹⁵N Utilization Optimization

The combined challenges of water scarcity and inefficient nitrogen use pose substantial barriers to sustainable agricultural development. Optimizing the coordinated regulation of water and nitrogen resources in fruit trees is essential for promoting water-saving agriculture in drylands. To establish a water and nitrogen collaborative management model for efficient resource utilization, this study conducted a 3-year field experiment examining different irrigation amount (W1: 4500 m³·ha⁻¹, W2: 6000 m³·ha⁻¹, and W3: 7500 m³·ha⁻¹) and nitrogen application rates (N1: 200 kg·ha⁻¹, N2: 300 kg·ha⁻¹, and N3: 400 kg·ha⁻¹), coupled with ¹⁵N isotopic labeling, to evaluate the impact of water and nitrogen regulation on the following: (i) the spatial distribution patterns of water and nitrogen in the root zone soil, (ii) dynamic characteristics of water and nitrogen across organs, and (iii) ¹⁵N absorption and utilization. The findings revealed that 20–80 cm depth was the key zone for water and nitrogen absorption by roots of pear. The W2 treatment met the optimal irrigation requirement for young pear tree roots and exhibited the optimal dynamic characteristics of water and nitrogen among the newly formed organs. At the end of the growth period, N3 treatment had the highest nitrogen content and the root system was the main organ for nitrogen absorption and storage. Water-saving irrigation coupled with optimized nitrogen application synergistically enhanced the nitrogen accumulation efficiency in fragrant pear. The W2N2 treatment exhibited the highest ¹⁵N absorption and utilization rate (40.79%), effectively promoting nitrogen absorption and assimilation, reducing nitrogen losses, and offering valuable insights for advancing sustainable practices in the fruit and forestry industries.