Carbon currency mediates <scp>AM</scp> fungal–microbe interactions

Abstract Arbuscular mycorrhizal (AM) fungi redistribute plant‐derived photosynthetic carbon (C) into the hyphosphere, where it mediates complex microbial interactions. This review synthesizes evidence suggesting that the hyphosphere may function as a dynamic micro‐marketplace, with fungal‐exuded C serving as a resource currency whose value is modulated by C form, lability and accessibility to different microbial guilds. We aim to integrate the surplus C hypothesis and biological market principles to propose how C acts as a resource currency and nutrient availability as its price regulator, potentially governing trade outcomes between AM fungi and soil microbes. Critically, we distinguish the hyphosphere micro‐market from classic biological market models by emphasizing that fungal C exudation is not necessarily under tight reciprocal control but rather emerges from plant source‐sink dynamics, creating a ‘capital pool’ that then enables conditional microbial transactions. Under nutrient limitation, conditional cooperative exchanges (e.g. C‐for‐P trades) can emerge though we note that such cooperation is conditional and may be less common than competitive interactions in undisturbed soils. Under C scarcity or nutrient surplus, cooperation collapses, shifting the system toward multi‐scale competition, from antagonistic interactions to network‐mediated allocation conflicts. This framework positions AM fungi as ecosystem engineers that allocate 10%–30% of host C to structure microbial communities and drive nutrient cycling, with implications for symbiotic stability and ecosystem functioning. Synthesis . Viewing the hyphosphere as a micro‐market unifies mechanisms of cooperation and competition when we explicitly recognize the market as a heuristic framework rather than assuming strict economic rationality. Future research should quantify real‐time C flows using coupled isotope tracing and microbial services to guide management of soil carbon economies for sustainable ecosystem outcomes.