Independent and differential effects of arbuscular mycorrhizal fungal composition and plant pathogens on plant traits and nitrogen uptake

Arbuscular mycorrhizal fungi (AMF), as microbial mutualists, interact with various microbial taxa, including pathogens, and significantly shape the ecology and evolution of their host species. However, how AMF and pathogens jointly or independently affect plant traits and nitrogen uptake remains unclear. Here, we conducted a factorial experiment with three AMF treatments (AMF-free-control, <i>Funneliformis mosseae</i>, and a mixture of AMF species of <i>F. mosseae</i> and <i>C. etunicatum</i>), four plant-pathogen pairs, each under two pathogen treatments (one pathogen and a pathogen-free control). After 65 days of growth, we measured AMF colonization, pathogen infection, plant functional traits and ammonium and nitrate uptake. Our findings reveal that AMF and pathogens independently influence plant traits and nitrogen uptake, with no observed interactions between them. Specifically, colonization by <i>F. mosseae</i> or a mixed AMF species reduced nitrate uptake and plant height, without affecting root traits or ammonium uptake. In contrast, pathogen infection enhanced acquisitive root traits, such as increased specific root length and area but did not impact shoot traits or nitrogen uptake. These results broaden our understanding of the tripartite interactions among plants, AMF and pathogens, offering insights into how plant-microbial relationships influence plant health, growth and nitrogen cycling.