Reciprocal transport of carbon and nitrogen between plants and the hyphosphere microbiome in a temperate grassland

Summary Despite its relevance for plant nutrient acquisition, the role of the hyphosphere in structuring fungal–bacterial networks and mediating carbon–nitrogen (C–N) exchange under field conditions remains unclear. We therefore examined reciprocal C–N fluxes along the plant–fungi–bacteria continuum and their link to microbial community assembly across grassland soil microhabitats. Sterilized compartmentalized ingrowth cores were exposed to soil in the field for 3 months. Plants were pulse‐labeled with 13 CO 2 and 13 C was traced into microbial biomarkers over 7 d. 15 N‐labeled urea was supplied to a hyphal compartment and isotope recovery in plants was quantified. We reconstructed microbial networks and visualized microscale isotope distributions. Plant‐derived 13 C was rapidly incorporated into fungi within 1 d and into hyphosphere bacteria after 7 d. Conversely, 16.9% of the initially added 150 μg 15 N was transferred from the hyphal compartment to plants. Microbial diversity declined from the rooted toward the hyphal‐dominated compartments, while networks reorganized with relatively more positive fungal–bacterial associations. Arbuscular mycorrhizal fungi act as primary channels of reciprocal C–N exchange, rapidly transporting newly fixed plant C into root‐distant soil zones. More broadly, soil fungi act as ecological filters shaping bacterial assembly and may foster increasingly positive associations with bacteria in hyphal‐dominated habitats.