Phosphorus reward mechanisms of an arbuscular mycorrhizal fungus and a dark septate endophyte to plant carbon allocation: Synergism or competition?

Combined inoculation with dark septate endophytes (DSEs) and arbuscular mycorrhizal fungi (AMF) has been shown to promote plant growth, yet the underlying plant-fungus interaction mechanisms remain unclear. To elucidate the nature of this symbiosis, it is crucial to explore carbon (C) transport from plants to fungi and nutrient exchange between them. In this study, a pot experiment was conducted with two phosphorus (P) fertilization levels (low and normal) and four fungal inoculation treatments (no inoculation, single inoculation of AMF and DSE, and co-inoculation of AMF and DSE). The ¹³C isotope pulse labeling method was employed to quantify the plant photosynthetic C transfer from plants to different fungi, shedding light on the mechanisms of nutrient exchange between plants and fungi. Soil and mycelium δ¹³C, soil C/N ratio, and soil C/P ratio were higher at the low P level than at the normal P level. However, soil microbial biomass C/P ratio was lower at the low P level, suggesting that the low P level was beneficial to soil C fixation and soil fungal P mineralization and transport. At the low P level, the P reward to plants from AMF and DSE increased significantly when the plants transferred the same amount of C to the fungi, and the two fungi synergistically promoted plant nutrient uptake and growth. At the normal P level, the root P content was significantly higher in the AMF-inoculated plants than in the DSE-inoculated plants, indicating that AMF contributed more than DSE to plant P uptake with the same amount of C received. Moreover, plants preferentially allocated more C to AMF. These findings indicate the presence of a source-sink balance between plant C allocation and fungal P contribution. Overall, AMF and DSE conferred a higher reward to plants at the low P level through functional synergistic strategies.