Age-Dependent Phosphorus Fractions Underpin Above - Belowground Nutrient Strategies in Plantation Forests

Increasing atmospheric nitrogen (N) deposition can stimulate tree growth, raising phosphorus (P) demand relative to supply and thereby intensifying relative P limitation. Yet how relative P limitation and above- and belowground resource-use strategies change during plantation development, and whether responses to chronic N enrichment vary with stand age, remain unclear. We integrated a synthesis of 926 observations from plantation forests across China with two co-located field experiments: a seedling P-addition bioassay and a 15-year N-addition experiment across a larch plantation chronosequence. Relative P limitation was strongest in young stands and weakened with age. This pattern coincided with coordinated shifts in resource-use strategies: leaf strategies shifted from conservative to acquisitive resource use, whereas root strategies shifted from self-reliant foraging to symbiotic acquisition. Chronic N addition further exacerbated relative P limitation, with young trees enhancing resorption of leaf nucleic acid P and lipid P and increasing reliance on rhizosphere secondary mineral P acquisition, whereas mature trees reallocated leaf P fractions. These findings link the age-related relaxation of P limitation to coordinated above- and belowground resource-use strategies and reveal that chronic N enrichment elicits age-specific adjustments in P acquisition, allocation and conservation, supporting age-specific management of plantation nutrient balance under global change.