Objectives Given the complex reality of the continuous rise in atmospheric CO₂ concentration and the scarcity of available phosphorus (P) in the soil of the south, the interaction of environmental factors on the symbiosis of arbuscular mycorrhizal fungi (AMF) and host Chinese fir (Cunninghamia lanceolata) was studied.

Methods One-year-old seedlings of two clones of Chinese fir, NO.020 and NO.061, were used as experimental materials in a three-factor coupling experiment. The three factors and their levels included: CO₂ concentration (C400, C600, C800), P supply level (P0.05, P0.5, P1), and AMF inoculation (−AMF, +AMF). The photosynthetic characteristics and the allocation of the assimilated carbon in shoot and root of Chinese fir clones were measured.

Results Under the interaction of environmental factors, the photosynthetic characteristics and carbon distribution of the two clones differed. NO.020 acquired P nutrient through AMF and maintained gas exchange to increase leaf carbon input and 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 RuBisCO significantly increased by 7.86%, while the content of PEPC significantly decreased by 9.85%. G_s and T_r decreased significantly by 24.3% and 24.32% respectively. Under the conditions of P0.05 and P0.5, the P_n and E_WUE of NO.020 leaves decreased significantly, while 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 translate 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 q_N increased significantly by 12.23% and 5.89% respectively. Under P1 treatment, +AMF treatment increased the 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 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 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.