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 (DSE) and arbuscular mycorrhizal fungi (AMF) has been shown to promote plant growth, yet the underlying plant-fungal interaction mechanisms remain unclear. To elucidate the nature of this symbiosis, it is crucial to explore carbon transport from plants to fungi and nutrient exchange between them. In this study, a 13carbon (C) isotope pulse labeling method was employed to quantify the C distribution ratio from plants to different fungi, shedding light on the mechanisms of nutrient exchange between plants and fungi. Soil and mycelial δ13C, soil carbon/nitrogen (C/N), and soil C/P ratios were higher at the low phosphorus (P) level than at the normal P level. However, soil microbial carbon/microbial phosphorus (MBC/MBP) was lower at the low P level, suggesting that 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 C source to the fungi, and the two fungal types synergistically promoted plant nutrient uptake and growth. At the normal P level, the root P content was significantly higher in AMF-inoculated plants than in DSE-inoculated plants, indicating that AMF contributed more than DSE to the P uptake of plants with the same C source. Moreover, plants preferentially allocated more C to AMF.1 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.