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ABSTRACT 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.
Zhang et al. (Mon,) studied this question.