Phosphorus availability governs the fate of photosynthetic carbon in the plant–soil–microbe system of <i>Pinus massoniana</i> under elevated <scp> CO ₂ </scp>

Summary The effects of elevated CO 2 (eCO 2 ) on tree growth and carbon dynamics may be constrained by soil phosphorus (P) availability. However, whether their interaction shapes the fate of recent photosynthetic carbon allocation along plant–soil–microbe compartments is rarely explored. Potted Pinus massoniana seedlings were grown under two levels of CO 2 (aCO 2 : 420 ppm and eCO 2 : 800 ppm) and four P treatments (0, 25, 50, and 100 kg P ha −1 yr −1 ) in Open‐Top Chambers, with 13 C labeling employed to trace allocation of recent carbon among plant organs, soil, and microbes. Biomass, structural, and non‐structural carbohydrates were also determined. Results showed that eCO 2 increased the allocation of 13 C to the root, while eCO 2 + P100 prioritized allocation to the leaf. eCO 2 consistently raised DO 13 C ( 13 C‐dissolved organic carbon), but its effect on MB 13 C ( 13 C‐microbial biomass carbon) was significant only at P50 and P100. Correlation analysis confirmed the carbon flow from leaf to stem to root and identified root 13 C as the key source for dissolved organic carbon and a critical driver for microbial carbon buildup under sufficient P. Our findings indicate that P addition modulated 13 C allocation among plant organs under eCO 2 and enabled microbes to efficiently utilize root‐derived carbon, demonstrating that P availability serves as a key regulator of carbon‐phosphorus exchange in the plant–soil–microbe system of P. massoniana .