Phosphorus availability governs the fate of photosynthetic carbon in the plant-soil-microbe system of Pinus massoniana under elevated CO2.

The effects of elevated CO2 (eCO2) 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 CO2 (aCO2: 420 ppm and eCO2: 800 ppm) and four P treatments (0, 25, 50, and 100 kg P ha-1 yr-1) in Open-Top Chambers, with 13C 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 eCO2 increased the allocation of 13C to the root, while eCO2 + P100 prioritized allocation to the leaf. eCO2 consistently raised DO13C (13C-dissolved organic carbon), but its effect on MB13C (13C-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 13C 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 13C allocation among plant organs under eCO2 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.