Ammonium-Nitrate Ratio and Soil Moisture Jointly Regulate Early-Spring Nitrogen Acquisition Through Root Architectural Plasticity in Apple

Early-spring nitrogen (N) acquisition in perennial fruit trees represents a critical physiological bottleneck determining vegetative-reproductive balance, as remobilized woody-tissue N supports initial growth but excessive depletion compromises reproductive performance. The interactive effects of N form (NH4+ or NO3-) and soil moisture on root morphological plasticity and whole-plant N allocation during this period remain poorly characterized. We subjected two-year-old apple trees to five NH4+/NO3- ratios under contrasting soil moisture regimes spanning budbreak to shoot elongation. Combined NH4+/NO3- supply, particularly at 25:75, optimized root architecture by maintaining high absorptive-to-structural root length ratios at moderate total root length, enhancing whole-plant total N accumulation by 59% and 21% over sole nitrate and ammonium, respectively, under moderate drought (50% FC). Balanced N supply decoupled carbon (C) and N partitioning, achieving the highest C-N allocation deviation index (0.68), facilitating preferential N enrichment in shoots while minimizing root C investment. Leaf nitrate reductase activity under drought reached maximum values under balanced supply despite reduced substrate availability, indicating that whole-plant photosynthetic status rather than substrate concentration determined assimilation capacity under water limitation. Partial least squares path modeling revealed soil moisture functions as a mechanistic switch determining N acquisition pathways: under drought, N accumulation was driven predominantly by root morphological quality (absorptive root proportion and specific root length; β=0.76), whereas under adequate moisture, absorptive root length became the primary driver (β=0.80). These findings demonstrate that coordinating NH4+/NO3- ratio with soil water availability provides a mechanistic basis for climate-adapted fertigation strategies supporting early-season N acquisition under intensifying spring drought.