Mycorrhizal Type and Soil Nitrogen Content Coregulate Foliar Nutrient Responses to Neighborhood Functional Dissimilarity in Subtropical Forests

Foliar nitrogen (N) and phosphorus (P) concentrations are of critical importance to plant productivity. Despite global declines in plant diversity, their effects on tree foliar N and P dynamics remain uncertain, especially under different mycorrhizal types and soil nutrient conditions. Based on a large biodiversity experiment in subtropical China, we assessed how neighborhood species richness and functional dissimilarity influence foliar N and P concentrations across 794 tree individuals, comprising three arbuscular mycorrhizal (AM) and five ectomycorrhizal (EcM) tree species, along natural soil total N gradients. At the neighborhood scale, foliar nutrients were jointly influenced by functional dissimilarity, mycorrhizal type, and soil N availability. Among dissimilarity metrics, wood density (WD) dissimilarity was the strongest predictor. Specifically, functional dissimilarity consistently increased foliar N and P concentrations in AM trees across the soil total N level, whereas its effects on EcM trees shifted from positive to negative with increasing soil total N content. These diversity-driven increases in foliar P concentration were further associated with enhanced tree growth. Our findings demonstrate that mycorrhizal type and soil N availability jointly mediate effects of neighborhood diversity on tree foliar nutrient status, with foliar P concentration playing a pivotal role in driving productivity responses to biodiversity in subtropical forests.