Nitrogen addition altered the plant-arbuscular mycorrhizal fungi network through reducing redundant interactions in an alpine meadow

Abstract

Interactions between plants and arbuscular mycorrhizal fungi (AMF) are pivotal in linking belowground soil nutrients with plant-derived carbon in terrestrial ecosystems. However, little is known about the effect of variation in soil nutrients (i.e., nitrogen enrichment) on the structure and phylogenetic characteristics of the mutualistic network. With a 7-year nitrogen addition experiment in an alpine meadow, we built networks for 20 plant species that all occur along the nitrogen addition gradient and their AMF by sequencing 314 root samples. We investigated the responses of association degree (i.e., the number of links of plant or AMF species), network connectance, nestedness, modularity and specialization to different concentrations of nitrogen addition. Phylogenetic signal was calculated to evaluate if the phylogenetically closely related plant species or AMF taxa share similar symbiosis partners. We found that nitrogen addition significantly reduced the plant and AMF association degree, with the generalist (with high association degree) and abundant AMF taxa in nature significantly decreasing association degree and relative abundance (sequencing reads). Therefore, the network connectance and nestedness decreased, but modularity increased with nitrogen addition. Phylogenetic signal of plants (i.e., phylogenetically related plants had similar AMF partners) was always stronger than that of AMF, but the signal vanished with nitrogen addition. The shifted soil properties after nitrogen addition, especially pH, played an important role in regulating the network structures. Our results discover a non-random plant-AMF network shaped by both phylogeny and soil environments in an alpine meadow, and suggest that global nitrogen addition may result in a non-random loss or switch of plant-fungal associations, changes in network structures and phylogenetic relationships, which may have ramification to nutrient exchange and symbiotic performance.