Shifts in plant nutrient acquisition strategies under nitrogen addition: a global meta-analysis

Background and aims Plants acquire nutrients by extending root length to explore soil resources and by metabolically exploiting the nutrients encountered. While global nitrogen (N) deposition has substantially enhanced aboveground productivity, it remains unclear how plants sustain this increase by coordinating changes in fine-root biomass and traits to restructure exploration–exploitation strategies. Methods We compiled fine-root trait data from 138 field experiments across 156 global sites and employed meta-analysis to quantify responses of fine-root biomass, morphological fine-root traits, nutrient uptake rate proxies, and mycorrhizal traits under N-only and N+P additions. Results N-only addition enhanced both exploration and exploitation capacities globally but reduced plant dependence on mycorrhizal fungi. In woody plants, fine-root biomass remained unchanged, but increased root tissue density (RTD, +11.9%) reduced root length density (RLD, –7.9%), while root nitrogen concentration (RN, +13.2%) and respiration rate (Rs, +21.5%) boosted exploitation capacity. Herbaceous plants adopted an acquisitive strategy, increasing fine-root biomass (+71.3%), specific root length (SRL, +67.8%), RN (+9.8%), and Rs (+24.6%), driving substantial RLD gains (+88.0%) and nutrient exploitation. N+P addition sustained mycorrhizal reliance and enhanced woody exploration (RLD +25.5%). Low baseline SRL predicted stronger biomass and RLD responses, identifying SRL as a key driver of belowground investment under N-only addition. Conclusions Our results reveal contrasting strategies between woody and herbaceous plants and highlight SRL as a critical predictor of root biomass responses. These findings provide new insights into how plants adjust nutrient acquisition strategies under N addition and inform predictions of ecosystem carbon–nutrient dynamics under global change.