Background and aims <p>Root traits are key predictors in shaping plant-fungal relationships. However, the potential influence of intraspecific root functional traits on rhizosphere fungal communities in response to hydrological gradients, remains poorly characterized. This study aims to investigate how intraspecific root functional traits influence rhizosphere fungal community assembly along hydrological gradients.</p> Methods <p>We conducted an in situ field investigation across reservoir riparian zones, quantifying intraspecific root traits, including morphological and architectural traits, and sequencing the rhizosphere fungal community of the dominant species <i>Cynodon dactylon</i> (L.) Pers.</p> Results <p>Our study revealed intraspecific divergence in <i>C. dactylon</i> root traits across the hydrological gradient, demonstrating a resource 'collaboration gradient' from a 'do-it-yourself' ('DIY') strategy characterized by high specific root length to an 'out-sourcing' strategy with thickened roots and elevated root tissue density. The shifts in root traits aligned with opposing fungal responses, with the relative abundance of predicted plant saprotrophs more closely associated with the 'DIY' axis at lower elevations and arbuscular mycorrhizal fungi (AMF) diversity positively associated with 'out-sourcing' traits at higher elevations. Root traits collectively demonstrated stronger explanatory power over AMF community assembly compared to general fungal communities, with root average diameter, fractal abundance and branching rate emerging as key predictors of symbiotic specificity.</p> Conclusion <p><i>C. dactylon</i> shifts root strategies from 'DIY' to 'out-sourcing' along hydrological gradients, promoting general fungi at low elevations and AMF at high elevations. Root diameter, fractal abundance, and branching rate critically shape AMF assembly, highlighting trait-linked fungal adaptation.</p> Graphical Abstract <p></p>