Phosphorus (P) availability and efficiency are especially important for plant growth and productivity. However, the sex-specific P acquisition and utilization strategies of dioecious plant species under different N forms are not clear.

This study investigated the responsive mechanisms of dioecious Populus cathayana females and males based on P uptake and allocation to soil P supply under N deficiency, nitrate (NO₃−) and ammonium (NH₄+) supply.

Females had a greater biomass, root length density (RLD), specific root length (SRL) and shoot P concentration than males under normal P availability with two N supplies. NH₄+ supply led to higher total root length, RLD and SRL but lower root tip number than NO₃− supply under normal P supply. Under P deficiency, males showed a smaller root system but greater photosynthetic P availability and higher leaf P remobilization, exhibiting a better capacity to adaptation to P deficiency than females. Under P deficiency, NO₃− supply increased leaf photosynthesis and P use efficiency (PUE) but reduced RLD and SRL in females while males had higher leaf P redistribution and photosynthetic PUE than NH₄+ supply. Females had a better potentiality to cope with P deficiency under NO₃− supply than NH₄+ supply; the contrary was true for males. These results suggest that females may devote to increase in P uptake and shoot P allocation under normal P availability, especially under NO₃− supply, while males adopt more efficient resource use and P remobilization to maximum their tolerance to P deficiency.