Abstract Plant invasion and nitrogen (N) deposition are escalating global change threats. Arbuscular mycorrhizal fungi (AMF) are increasingly recognized as critical mediators of plant invasion success, largely through their role in enhancing host nutrient acquisition. Notably, AMF preferentially transfer ammonium (NH 4 + ) over nitrate (NO 3 − ) to host plants, a physiological trait that may differentially favour invasive species depending on soil N‐form availability. Given that anthropogenic activities are altering soil NH 4 + /NO 3 − ratios worldwide, understanding AMF‐mediated N‐form effects on competitive invasion dynamics is critical for predicting invasion outcomes under global change. To examine how AMF differentially mediate competition and N acquisition between invasive and native plants under contrasting N forms, we conducted a factorial common garden experiment using four Asteraceae forbs (two invasive and two native species). The experimental design included: two N forms (NH 4 + vs. NO 3 − ), three AMF treatments (−AMF without inoculation, +AMF roots + mycelia access to added N and mycelium mycelia‐only) and two competition levels (single species vs. mixture). The invaders had a competitive advantage under both N forms, driven by strengthening their own AMF associations while inhibiting AMF colonization with native plants. The facilitation of AMF was stronger under NO 3 − than under NH 4 + , although the mycelium treatment confirmed that AMF contributed more under NH 4 + than NO 3 − . The partial least squares path models revealed that invasive plants with greater competitive ability exhibited stronger plastic responses in root–shoot allocation and nutrient uptake following AMF inoculation. Synthesis . These findings reveal that AMF‐mediated plant invasion is dependent on soil N form. The greater benefit that invasive plants derive from AMF relative to native plants appears to reflect the combined effects of both the enhanced mutualism hypothesis and mutualism disruption hypothesis . This N‐form dependency carries important implications for understanding invasion dynamics in heterogeneous landscapes, where agricultural runoff and atmospheric deposition are increasingly influencing soil N form in many grassland systems. Our results advance the understanding of context‐dependent AMF–plant interactions and underscore the necessity of integrating N‐form dynamics into invasion ecology frameworks. Read the free Plain Language Summary for this article on the Journal blog.
Su et al. (Fri,) studied this question.