Deciphering the mycorrhizal nexus: Root economic trade‐offs shape arbuscular mycorrhizal fungal assembly dynamics to enhance crop productivity

Whether arbuscular mycorrhizal fungal (AMF) symbionts are more likely to form in low‐ or high‐fertility soils has been debated for decades. The symbiotic efficiency of AMF on plants is determined not only by a trait‐based framework, such as root economics space (RES), but also by the environmental adaptability and the assembly processes of AMF communities. However, it remains unknown how RES regulates AMF community assembly and, in turn, affects AMF per cent colonization and plant productivity. We characterized RES based on maize root traits across a long‐term soil fertility gradient. In parallel, we explored the assembly processes of AMF generalists and specialists and elucidated how RES regulates AMF percent colonization. Our results indicated that carbon inputs and lime amendments enhanced the RES symbiosis gradient by improving soil fertility. AMF generalists were assembled primarily through stochastic processes, whereas AMF specialists were shaped by deterministic processes. We further revealed the dual mechanisms by which the symbiosis gradient regulated plant productivity. Under low‐nutrient conditions, a low symbiosis gradient increased the determinism of AMF specialists, ultimately enhancing plant productivity through increased AMF percent colonization. In contrast, under high‐nutrient conditions, a high symbiosis gradient increased stochastic assembly of AMF generalists, thereby enhancing plant productivity. Synthesis. Our research provides new insights into plant–AMF interactions, highlighting their role in optimizing crop productivity and developing sustainable agricultural practices.