Richness and Composition of Mycorrhizal Fungi Varies by Flood Level and River Basin in Oligotrophic Amazonian Seasonally Flooded Forests

Mycorrhizal fungi play a key role in supporting plants in nutrient-poor environments, yet their diversity and distribution in Amazonian floodplain forests remain poorly understood. Here, we assessed the presence of mycorrhizal fungi across three seasonally flooded by black water forest (igapó) sites in Central Amazonia (RDS Uatumã, RDS Rio Negro, and PARNA Jaú) using DNA metabarcoding of roots. We examined how flood regimes and locality influence the richness and composition of mycorrhizal fungi and, particularly, for ectomycorrhizal fungi (ECM). We found 568 mycorrhizal ASVs, including 307 ECM and 238 arbuscular mycorrhizal fungi. Mycorrhizal richness was higher in sites that experienced prolonged inundation. However, ECM richness was more affected by site than flood level. Community composition varied across sites and flood levels, with distinct fungal assemblages associated with different host tree species. Our results suggest that a complex interplay of flood dynamics, local characteristics, and plant-fungal interactions shapes the structure of mycorrhizal communities in igapós. Despite seasonal flooding, these forests harbor a diverse and specialized mycorrhizal biota, including many ECM lineages rarely documented in tropical wetlands. This study contributes with novel insights into Amazonian belowground biodiversity and highlights the importance of topographic and hydrological heterogeneity in maintaining adapted fungal diversity and function. Understanding these patterns is critical for forecasting the ecological impacts of climate-driven changes to Amazonian flood regimes.