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Ammonium-Nitrate Ratio and Soil Moisture Jointly Regulate Early-Spring Nitrogen Acquisition Through Root
Zengyuan Li1, Jiahuan Liu2, Yu Song1
1State Key Laboratory of Nutrient Use and Management, College of Resources and Environmental Sciences, National Academy of Agriculture Green Development, China Agricultural University, Beijing 100193, China.
Abstract:
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.
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