Absorptive root traits shape phosphorus acquisition strategies defined by trade-offs between phosphorus uptake and utilization under soil nutrient limitation and drought stress

Background and Objectives Nutrient limitation and drought stress often co-occur and jointly influence tree phosphorus (P) acquisition and variation in absorptive root traits. However, it remains unclear whether interspecific variation in P acquisition strategies under combined stress reflects a trade-off between P uptake rate (PUR) and P utilization efficiency (PUE), and how this variation is associated with absorptive root morphology and P uptake kinetic characteristics. Methods Seedlings of four broad-leaved deciduous tree species, Quercus variabilis, Prunus davidiana, Acer truncatum and Koelreuteria paniculata, were grown for eight weeks in a 3 × 3 factorial experiment with three nutrient limitation levels and three drought severity levels. We then measured total biomass, PUR, PUE, P uptake kinetic characteristics (Vmax, Km and α), and absorptive root morphological traits (root order ≤ 3), including specific root length (SRL), specific root surface area (SRA), root diameter (RD) and root tissue density (RTD). Results Species exhibited distinct P acquisition strategies under nutrient limitation and drought stress. A. truncatum and K. paniculata showed a more acquisitive strategy, with higher PUR, greater SRL and SRA, and lower RD and RTD. In contrast, Q. variabilis and P. davidiana showed a more conservative strategy, with higher PUE and denser absorptive roots. Among P uptake kinetic characteristics, α showed the clearest association with these interspecific strategies. Conclusion We identified a trade-off between PUR and PUE under combined nutrient limitation and drought stress, with tree species differing in the extent to which they relied on uptake versus utilization. These contrasting acquisitive and conservative strategies were associated with variation in absorptive root morphology, with α providing the clearest kinetic link, consistent with a root economic framework for P acquisition under stress.