Objectives Given the continuous increase in atmospheric CO₂ concentration and the widespread scarcity of available phosphorus (P) in southern soils, this study investigated the interactive effects of environmental factors on the symbiosis between arbuscular mycorrhizal fungi (AMF) and their host, Chinese fir (Cunninghamia lanceolata).

Methods One-year-old seedlings of two Chinese fir clones (NO.020 and NO.061) were used in a three-factor factorial experiment. The factors included: CO₂ concentration (C400, C600, C800, μmol/mol), P supply level (P0.05, P0.5, P1, mmol/L), and AMF inoculation (−AMF, +AMF). Photosynthetic characteristics and carbon allocation between shoots and roots were systematically analyzed.

Results  Significant differences in photosynthetic traits and carbon allocation were observed between the two clones under interacting environmental conditions. Clone NO.020 acquired P nutrient through AMF and maintained gas exchange, increasing carbon allocation to leaf to maximize carbon benefits. Under the +AMF treatment, the biomass distribution of NO.020 was transferred from the root system to the aboveground leaf part. The content of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) significantly increased by 7.86%, while the content of phosphoenolpyruvate carboxylase (PEPC) significantly decreased by 9.85%. stomatal conductance (G_s) and transpiration rate (T_r) decreased significantly by 24.3% and 24.32% respectively. Under the conditions of P0.05 and P0.5, the net photosynthetic rate (P_n) and water use efficiency (E_WUE) of NO.020 leaves decreased significantly, while intercellular CO₂ concentration (C_i) increased significantly by 31.79% under the +AMF-P0.05 treatment compared with the +AMF-P0.5 treatment. For NO.061, carbon investment did not smoothly transform into photosynthetic gain. It responded to nutrient limitations by reducing root investment and enhancing photoprotective capacity. The +AMF treatment significantly reduced the root system of NO.061 by 13.10% and significantly increased the leaf biomass by 11.07%. The contents of RuBisCO and PEPC also decreased significantly by 5.95% and 2.26% respectively, P_n decreased significantly by 33.0%, while C_i and non-photochemical quenching coefficient (q_N) increased significantly by 12.23% and 5.89% respectively. Under P1 treatment, +AMF treatment increased the photochemical quenching coefficient (q_P) of leaf NO.061. Under P0.05 and P0.5 treatments, the q_P decreased under the +AMF treatment, but the difference was not significant. Increasing the concentration of CO₂ can enhance the photosynthesis of plants, but there is a concentration threshold. The actual photochemical efficiency of photosystem Ⅱ Y_(Ⅱ) of the seedlings of the two clones of Chinese fir reached the peak under the C600 treatment. Under the C800 treatment, the G_s, T_r of NO.020 and C_i of NO.061 decreased significantly. Among them, the q_P of NO.020 under the +AMF-C600 treatment was 25.94%−27.93% higher than that of the other treatments. The symbiotic benefits of the seedlings of the two clones of Chinese fir were jointly affected by the supply of P level and the CO₂ concentration: under the P0.5 treatment, the infection rate showed a trend of first increasing and then decreasing with the increase of CO₂ concentration, while under the P1 treatment, it increased with the increase of CO₂ concentration.

Conclusions Significant genotypic differences exist in photosynthetic characteristics and carbon allocation between Chinese fir clones. These differences are regulated by complex environmental interactions. NO.020 adopts a “high investment-high return” growth strategy, while NO.061 exhibits a “conservative-stress resistant” strategy. Elevating CO₂ to 600 μmol/mol shows a fertilization effect, further increase reduces stomatal conductance and photochemical efficiency in NO.020, and also decreases carbon sequestration capacity in NO.061. Under elevated CO₂, the promoting effect of AMF on photosynthesis depends on phosphorus supply. Under normal phosphorus supply, the symbiotic benefit is significant. Under low phosphorus stress, the benefit weakens or even becomes inhibitory. Root colonization rate increases with CO₂ concentration under normal phosphorus supply. Under low phosphorus conditions, it peaks at the C600 treatment.