Arbuscular mycorrhizal interactions and nutrient supply mediate floral trait variation and pollinator visitation

Floral traits, including floral display and nutritional rewards from pollen and nectar, drive pollinator visitation. Even within a single plant species, environmental factors can influence the quality and quantity of floral resources. Yet, the ecological interactions driving this variation in floral resources, especially those belowground, remain unknown. Here, we investigate how soil microbial community composition and nutrient availability, specifically distinct arbuscular mycorrhizal fungi (AMF) species and phosphorus (P) supply, affect plant growth, AMF traits, floral traits, and how that, in turn, affects bee visitation. We found that increased AMF richness of functional diversity enhanced floral display (flower size and number) and rewards (nectar volume and pollen protein) and increased bee visitation. Using structural equation modeling, we found that AMF associations could boost bee visitation by enhancing flower size. However, trade-offs occur; flower size correlates negatively with root colonization but positively with hyphal length, suggesting that AMF traits drive the effects of AMF on flower growth. Overall, the effect of AMF on floral traits and bee visitation was not homogenous; instead, AMF trait differences interact with P supply, resulting in varying effects on floral traits and subsequently bee foraging dynamics. These results highlight that focusing on beneficial belowground interactions could provide an opportunity to bolster bee visitation.