Grazing reshapes plant nitrogen use trade-off between soil uptake and internal remobilization

Perennial plants rely on two key ecological strategies to supply nitrogen (N) for photosynthetic tissue construction: root N remobilization (internal cycling) and soil N uptake. How plants trade-off these strategies determines the economy of N use and is likely context-dependent. Yet, it is rarely quantified in situ across growth stages and under varying management, limiting the mechanistic understanding of how plants rebalance N-use trade-offs to adapt to grazing disturbance. To address this gap, we conducted an in situ experiment using ¹⁵N isotope labelling techniques in a semiarid grassland to quantify the contributions of these two strategies at two plant growth stages under nil, summer or autumn grazing. We found that the plant relied more on soil N uptake (58%) during the regreen stage, but more on N remobilization (57%) during the rapid-growth stage. Grazing, especially in autumn, significantly leaned N use trade-off toward conservative N remobilization. The shift in plant N use strategies was driven primarily by plant growth demand and soil N supply, but not root N stock. Plant N use trade-off leaned toward internal cycling when source traits were below or sink traits were above the thresholds, for example, soil N availability <9.0 mg kg^-1 or shoot carbon content >41.88%. Our study highlights the importance of the N resource-sink relations in mediating the trade-off in plant N use strategies, buffering grazing stress to grassland plants. These findings provide a mechanistic basis for improving ecosystem N cycling modeling and adaptive grazing management.