Abstract: Plants adapt to the changes of nitrogen and water availability by balancing nitrogen use efficiency (NUE) and water use efficiency (WUE) especially in grasslands, where water and nitrogen are limiting plant growth. While NUE-WUE relationship has received considerable attention, the plasticity bias between them remains unexplored. Here, we define this plasticity bias as the asymmetry in plasticity magnitude between NUE and WUE. In an experiment simulating nitrogen deposition and drought in a temperate grassland in northern China, we measured NUE, WUE and ten functional traits of six common plant species, evaluated the NUE-WUE plasticity bias, and identified the underlying key traits. Under either nitrogen addition or drought, the NUE-WUE plasticity bias in most species consistently exhibited a WUE-favoring direction, with an increased intensity. The relationships between the NUE-WUE plasticity bias and functional traits were strengthened under nitrogen addition but became non-significant under drought. Among the traits investigated, leaf nitrogen content (LNC) was identified as the key driver influencing the NUE-WUE plasticity bias. This study captures the dynamic imbalance of resource use by quantifying plasticity bias, moving beyond static correlation analysis. We emphasize the critical role of above-ground traits (such as LNC) in driving plant adaptive strategies, and suggest that future predictive models should incorporate both above- and below-ground traits to comprehensively capture whole-plant resource coordination under environmental change.

Key words: Functional traits, Global change, Leaf carbon to nitrogen ratio, Resource utilization efficiency, Semi-arid grassland, Stable isotope, Tradeoff