Variability in apple tree fine-root phenology, amount and spatial distribution in response to irrigation and nitrogen application levels under drip fertigation

Fine roots with high phenotypic plasticity are crucial for fruit crop adaptation and orchard management, yet their spatiotemporal dynamic responses to irrigation and nitrogen regimes under field conditions remain unclear. Based on a long-term apple fertigation trial with two irrigation levels (85% (W1) and 100% (W2) of field capacity) and four nitrogen rates (0 (N1), 120 (N2), 240 (N3) and 360 (N4) kg N ha−1), this study investigated fine-root production (FRP) and mortality (FRM) dynamics using minirhizotrons during 2020–2021, and evaluated how aboveground growth and soil environmental factors (moisture, temperature and nitrate-N) regulate their distribution and temporal variation. Results showed that annual FRP and FRM were 21.19–33.94% and 19.51–81.77% higher under W2 than W1 and both exhibited a unimodal response to nitrogen level, peaking at N3 in both years. In the dry year, significant irrigation-nitrogen interactions emerged where N-induced stimulation of FRP and FRM was decoupled under W1. FRP rate showed distinct bimodal and unimodal phenological patterns across years and treatments, co-regulated by aboveground vegetative and reproductive growth and soil water-nitrogen conditions. FRM rate peaked at leaf fall, likely due to carbohydrate depletion. Water deficit and N3 enhanced deep-root production to mitigate shallow-layer resource competition. Random forest model analysis revealed shoot growth dominated fine-root dynamics in the 0–76 cm profile, while soil temperature primarily regulated mortality in the 0–38 cm profile. This study clarifies the spatiotemporal regulation of fine roots under irrigation and nitrogen manipulation and underscores the coordination of above- and belowground processes in apple orchards.