Combined effects of arbuscular mycorrhizal fungi and nutrient addition on the functional traits of Phragmites australis

Arbuscular mycorrhizal fungi (AMF) can enhance terrestrial plant growth by promoting nutrient uptake. However, the effects of interactions between AMF and nutrient inputs on plant functional traits and their trade-offs remain poorly understood. In this study, a pot experiment was conducted using Phragmites australis as the host species, with AMF inoculation and non-inoculation treatments under three levels of nitrogen addition and two levels of phosphorus addition. The results showed that AMF inoculation significantly increased AMF colonization in the roots of P. australis. Under unfertilized conditions, AMF significantly promoted plant morphological development and biomass accumulation. Nitrogen addition primarily enhanced aboveground growth, while phosphorus addition significantly stimulated root system development. Regarding photosynthetic traits, AMF inoculation significantly increased carotenoid content, whereas phosphorus addition significantly reduced net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) by 18.5%, 25.8%, and 28.2%, respectively. Furthermore, AMF inoculation increased leaf nitrogen and phosphorus contents but reduced stem nitrogen content and the N:P ratios in fine roots, rhizomes, and stems. Trait network analysis revealed that AMF inoculation shifted the central traits from leaf biomass to rhizome biomass, indicating a transition in hub traits from aboveground to belowground organs. These findings suggest that AMF inoculation significantly promotes the growth of P. australis although the magnitude of this benefit is modulated by soil nitrogen and phosphorus availability, with stronger effects observed under nutrient-poor conditions. This study advances our understanding of nutrient–AMF–plant interactions in wetland ecosystems and provides a theoretical foundation for ecological restoration and nutrient management in vulnerable aquatic habitats.