Soil pH and organic phosphorus co-shape the diversity and assembly processes of arbuscular mycorrhizal fungal community in subtropical broadleaved forests

Abstract

Arbuscular mycorrhizal (AM) fungal represent a vital pathway for plant nutrient acquisition in soils, especially in nutrient-limited subtropical broadleaved forests where AM fungi extensively colonize roots. However, critical knowledge gaps persist regarding how soil phosphorus and nitrogen gradients (including organic and inorganic forms) influence the structure and assembly mechanisms of AM fungal communities in subtropical broadleaved forests. We conducted field sampling of Schima superba rhizosphere soils (typically AM tree species), analyzing the concentrations of key soil nutrient forms (e.g., available phosphorus, organic phosphorus, ammonium, nitrate) and the AM fungal community structure via high-throughput sequencing of the 18S rRNA gene. Correlation revealed that AM fungal biomarkers (NLFA 16:1ω5) increased with soil pH. AM fungal richness exhibited a significant quadratic relationship with soil organic phosphorus (OP) concentrations and was negatively correlated with the available phosphorus (AP) to OP ratio, suggesting that a higher proportion of readily available phosphorus suppresses AM fungal diversity. Community structure (indicated by NMDS1) was primarily influenced by total phosphorus (TP) concentrations and pH, while network complexity was significantly associated with AP and NH₄⁺-N. The assembly of AM fungal communities was dominated by deterministic processes, whose influence decreased with increasing soil OP and TP concentrations. Random forest analyses and structure equation modeling (SEM) demonstrated that the structure and assembly of AM fungal communities were jointly shaped by soil pH and P dynamic (especially OP and TP). Overall, these findings elucidate that soil pH and P forms (particularly OP) are pivotal factors governing the structure and assembly processes of AM fungal communities in subtropical broadleaved forests, advancing our understanding of AM fungal community variation in nutrient-poor environments.