Tree species-dependent effects of afforestation on soil fungal diversity, functional guilds and co-occurrence networks in northern China

Afforestation exerts a profound impact on soil fungal communities, with the nature and extent of these changes significantly influenced by the specific tree species selected. While extensive research has addressed the aboveground ecological outcomes of afforestation, the nuanced interactions between tree species and soil fungal dynamics remain underexplored. This study investigated the effects of afforestation with Caragana microphylla (CMI), Populus simonii (PSI), and Pinus sylvestris var. mongolica (PSY) on soil fungal diversity, functional guilds, and co-occurrence networks, drawing comparisons with neighboring grasslands. Our findings reveal a significant increase in soil fungal Chao1 richness following afforestation, though the degree of enhancement varied across tree species. Specifically, CMI and PSI forests showed notable increases in fungal richness, whereas the response in PSY forests was comparatively modest. Saprotrophic fungal groups, integral to organic matter decomposition, showed a substantial increase across all afforested sites, with CMI forests exhibiting an impressive 205.58% rise. Conversely, pathogenic fungi, which can negatively impact plant health, demonstrated a marked decrease within plantation forests. Symbiotic groups, particularly ectomycorrhizal fungi, were notably enriched solely in PSI forests. Co-occurrence network analysis further indicated that afforestation alters fungal network complexity: CMI forests displayed increased network interactions, while PSI and PSY forests exhibited a reduction in network connectivity. Soil bulk density and organic carbon content emerged as key factors influencing network complexity, whereas tree species identity played a crucial role in shaping soil fungal community composition. Collectively, these results emphasize the importance of adopting a species-specific strategy for afforestation to optimize soil fungal diversity and network structure, ultimately enhancing the ecological resilience and sustainability of forest plantation ecosystems.