Co-symbiosis of arbuscular mycorrhizal fungi (AMF) and diazotrophs promote biological nitrogen fixation in mangrove ecosystems

Nitrogen (N) is the most critical nutrient that affects the establishment and stability of mangrove ecosystems. Despite the importance of biological nitrogen fixation (BNF) mediated by diazotrophs in mangrove ecosystems, current studies largely overlook the influence of arbuscular mycorrhizal fungi (AMF) on diazotrophs-driven BNF process in the N-limited ecosystems. Here, we conducted a comprehensive study on root-associated AMF and their interspecific interactions with diazotrophs in mangrove ecosystems using high throughput sequencing methods. Our results showed that the richness/diversity of diazotrophs had significant positive correlations with AMF community in mangrove rhizosphere. BNF rate in the rhizosphere of all mangrove species increased with the increasing of positive co-occurrence between AMF and diazotrophs, suggesting that the positive cooperation may promote N fixation efficiency in mangrove ecosystems. We also identified the potential keystone taxa (Zi > 2.5 or Pi > 0.62) and determined their crucial implications for the ecological processes of N fixation in mangrove ecosystems. The random forest analysis further indicated that the AMF keystone taxa were the most important predictors of BNF in mangrove rhizosphere. In addition, path analysis indicated that the alpha-diversity of AMF and diazotrophs communities, N-related enzymes and sediment nutrition components (e.g., TC, TN, Fe) were the main factors driving BNF process in mangrove ecosystems. This study provides novel insights into the interactions between AMF and diazotrophic communities during BNF process and expands our knowledge of AMF ecological functions of N cycle in mangrove ecosystems.