Arbuscular mycorrhizal fungi- linked microbial processes in soil nitrogen cycling

Arbuscular mycorrhizal fungi (AMF) create an extended plant-soil interface (referred to as mycorrhizal hyphosphere) where specific microbial interactions shape key steps of the nitrogen (N) cycle. Extraradical hyphae of the AMF host diverse microbes that help mineralize organic substrates, regulate ammonium and nitrate transformations, and enhance N retention. These processes allow plants to access both inorganic N and the N released from complex organic compounds. This review synthesizes current evidence for AMF-microbe interactions in relation to mineralization, nitrification, denitrification, and (di)nitrogen fixation. It also highlights unresolved questions, such as when AMF transition from facilitating to competing for N, how they access stabilized organic N pools, and how the carbon:nitrogen:phosphorus (C:N:P) nutrient stoichiometry of the soil environment constrain AMF-mediated N transfer to plants. Furthermore, we discuss how AMF-centered pathways can contribute to more circular N flows in agroecosystems by promoting tighter internal N cycling through microbial immobilization, turnover, and subsequent transfer to plants via AMF hyphae. By integrating spatial, microbial, and stoichiometric perspectives, this review provides a mechanistic framework for understanding AMF-driven N dynamics and their potential role in enhancing N use efficiency in managed and natural systems.