Long-Term Inorganic Nitrogen Fertilization Drives a Trade-Off in the Soybean Symbiotic Network From Low-Loss Fixation...

The productivity and sustainability of legume crops are highly dependent on their tripartite symbiotic system with rhizobia and arbuscular mycorrhizal fungi (AMF), a system currently facing significant pressure from long-term excessive nitrogen (N) fertilization. However, how long-term N input and host niche selection jointly regulate the structure and function of this tripartite symbiotic system remains poorly understood. Using a soybean pot experiment with soils collected in 2022 from a 24-year field experiment (winter wheat-summer maize rotation, receiving annual urea at 0, 200, 400, 600 kg N ha^-1 year^-1 since 1998), we systematically elucidated these mechanisms. The results demonstrate that host niche selection is the dominant driver structuring the core symbiotic network. This manifests as a progressive, stringent homogeneous selection for rhizobia from soil to nodules, and as dispersal limitation for AMF, imposed by strong physical filtration at the root epidermis. Long-term N input nonlinearly disrupted this host-dominated framework. Specifically, excessive N fertilization shifted rhizobial community assembly from deterministic to stochastic dominance, weakened their cross-kingdom synergy with AMF, and triggered a transition in the systemic N-cycling pathway. This transition moved from an efficient, low-loss internal symbiotic N-fixation mode to a high-loss-risk external N metabolism mode. This functional trade-off ultimately compromised the system's nutrient accumulation and retention capacity, offering a mechanistic explanation for how excessive N fertilization drives agroecosystems from symbiosis-dependence to fertilizer-dependence. These findings demonstrate that optimizing N management sustains nutrient retention and productivity by preserving the host-shaped symbiotic network, offering a reference for reducing fertilizer dependence and improving the sustainability of legume production.