Soil properties and plant functional traits drive soil bacterial and fungal colonization in tropical karst and non-karst natural forests along the elevation gradient

There are still many limitations in understanding the effects of the abiotic and biotic factors on soil bacterial and fungal communities in different lithological areas along the elevation gradient. In this study, plant biodiversity, functional traits, climatic factors, soil properties, and soil fungal and bacterial diversities and compositions were simultaneously measured in tropical karst and non-karst natural forests along elevation profiles in Southwestern China. Our results indicated plant taxonomic diversity, functional diversity, and phylogenic diversity were higher in karst areas than that in non-karst areas. Plant taxonomic diversity, functional diversity and phylogenic diversity increased with elevation increasing. The community-weighted mean leaf calcium and nitrogen concentrations in karst regions were significantly higher than that in non-karst regions. With the elevation increased, the plant functional traits changed from acquisitive traits to conservative traits. Soil bulk density in karst areas was lower than that in non-karst areas, while soil organic carbon was greater in karst areas as compared to non-karst areas. Soil calcium concentrations in karst areas were significantly higher than those in non-karst areas. Both fungal and bacterial diversities increased with elevation, and were generally higher in karst areas than those in non-karst areas. Relative abundance of Proteobacteria and Acidobacteria in karst areas were lesser than in non-karst areas, whereas the opposite pattern was observed for Actinobacteria. Boosted regression tree models indicated that the bacterial and fungal diversities and compositions were primarily affected by soil properties, followed by plant functional traits, biodiversity, and climatic factors. Structural equation models indicated that soil properties and functional traits directly impacted microbial distributions, whereas climatic factors and plant biodiversity only indirectly influenced these distributions through plant functional traits or soil properties. Our results highlighted soil calcium was a key regulator of bacterial and fungal distributions in karst and non-karst regions. The increased soil metal elements and plants with conservative functional traits generally increased bacterial and fungal diversities.