Sex‐Specific Root Strategies Drive Asymmetric Nitrogen Transfer via Common Mycorrhizal Networks Under Nitrogen Deficiency in Populus cathayana

ABSTRACT Common mycorrhizal networks (CMN) formed by arbuscular mycorrhizal (AM) fungi are critical pathways for plant nutrition and interplant nutrient transfer. However, their role in mediating sexually asymmetric interactions in dioecious plants remains poorly understood, especially under nitrogen (N) deficiency. Using in vivo 15 N leaf‐labeling in Populus cathayana saplings, we quantified N transfer via CMN between sexes and linked them to root traits. We found that intersexual pairs facilitated CMN‐mediated N transfer from male to female saplings; this transfer was markedly amplified under low N conditions. When N availability shifted from sufficient to deficient conditions, males shifted from a conservative strategy to an AM fungi‐dependent ‘outsourcing’ strategy (characterized by higher mycorrhizal colonization rates), whereas females transitioned from a relatively weak root foraging strategy to an enhanced one (with greater specific root length and specific root area). This strategic divergence promoted sexually asymmetric N transfer via CMN, leading to optimized nutrient use efficiency at the population level. These results highlighted a previously unrecognized role of CMN in facilitating sexually asymmetric nutrient interactions, offering a mechanistic framework to improve both productivity and sustainability in dioecious plantations on nutrient‐poor soils.