Ectomycorrhizal fungi in deep soils: current knowledge, functional roles, and future directions

Ectomycorrhizal (ECM) fungi are key regulators of nutrient acquisition, plant performance, and carbon cycling in forest ecosystems, yet their ecological roles have been investigated predominantly in surface soil horizons. Increasing evidence indicates that ECM fungi extend well into deep mineral layers, where they form vertically stratified communities with distinct taxonomic composition and functional traits. In this review, we synthesize current knowledge on the distribution, functional ecology, and ecosystem implications of ECM fungi in deep soils. Field studies reveal pronounced vertical turnover in ECM communities, driven by gradients in resource availability, oxygen concentration, and physicochemical constraints. Advances in molecular approaches, including long read metabarcoding technologies, have improved taxonomic resolution at depth, although distinguishing active fungi from persistent DNA remains a major challenge. Functionally, deep-soil ECM fungi might contribute to the vertical translocation of plant-derived carbon, the formation of mineral-associated organic matter, and the mobilization of nitrogen and phosphorus pools that are otherwise poorly accessible. Their extensive mycelial networks may also facilitate water acquisition and hydraulic redistribution, potentially enhancing plant drought resilience. These processes integrate surface productivity with subsurface biogeochemical dynamics and may influence long-term carbon storage and forest stability. Despite growing recognition of their importance, deep-soil ECM fungi remain underrepresented in experimental studies and ecosystem models. We highlight key methodological limitations and propose integrative research approaches aimed at quantifying activity, turnover, and plant-fungal trait interactions along soil depth. Incorporating deep mycorrhizal processes into ecological and Earth system frameworks is essential for improving predictions of forest responses to future environmental change.