Optimizing apple yield and water productivity through depth-specific fine-root architecture and turnover under water-nitrogen synergy

As climate change and resource scarcity intensify, optimizing water and nitrogen management is crucial for achieving sustainable intensification of apple production. Improving the efficient utilization of soil resources by regulation crop root systems represents could achieve synergistic improvements in both yield and resource-use efficiency. However, the mechanisms by which optimized irrigation and nitrogen fertilization reshape root architecture and turnover to drive yield and water productivity (WP c ) remain elusive. A multi-year field experiment (2017–2021) was conducted in an apple orchard under drip fertigation, comprising two irrigation levels, 85% (W 1 ) and 100% (W 2 ) of field capacity, and four nitrogen rates, 0 (N 1 ), 120 (N 2 ), 240 (N 3 ) and 360 (N 4 ) kg ha -1 . Irrigation and nitrogen application significantly influenced fine-root phenology; deficits in both factors suppressed the magnitude and number of fine-root production (FRP) peaks. Annual FRP was 21.19–33.94% higher under W 2 than W 1 and exhibited a unimodal response to nitrogen, peaking at N 3 . Fine-root turnover (FRT) responded to nitrogen application by adopting a depth-specific strategy, with the surface layer exhibiting the fastest FRT under N 3 and the deeper layer under N 4 . In the dry year, a significant irrigation-nitrogen interaction emerged where N-induced stimulation of FRT was decoupled under W 1 . Regression analysis revealed that FRP and FRT in deeper soil layers (below 38 cm) were key for improving yield and WP c . FRP exhibited a positive linear correlation with ET, yield and WP c . FRT exhibited a concave downward relationship with yield and WP c , which reflects the precise trade-offs between carbon investment and resource acquisition. Notably, FRT showed no consistent association with ET. Overall, W 1 N 3 promoted deeper root distribution and moderately increased FRP and FRT, thereby supporting aboveground physiological functions while significantly lowering ET, ultimately maximizing WP c . These findings provide a new research perspective on strategies to optimize orchard yield and WP c .