Contrasting Patterns of Soil Chemistry and Vegetation Cover Determine Diversity Changes of Soil Phototrophs Along an Afrotropical Elevation Gradient

Soil phototrophic microbes play key roles in many ecosystem functions, including nutrient cycling, water absorption and retention, substrate weathering and soil stabilization, as well as colonization and persistence of other organisms. Knowledge about the diversity and biomass of soil phototrophs remains limited, especially in tropical forests and savannas. Here, we investigate changes in the diversity and abundance of soil phototrophs across the 4-km elevation gradient on Mt. Cameroon, Africa, from tropical forests (0-2300 m) to treeless savanna (2300-3600 m) and afroalpine vegetation (3600-4000 m). We evaluated the role of soil chemistry and vegetation cover in shaping phototrophic diversity patterns using soil, tree and herb census data from 224 permanent plots. Cyanobacteria from Chroococcales accounted for 65% of the species richness and > 70% of the biovolume. The highest phototrophic diversity and biovolume were recorded in treeless savanna and afroalpine vegetation, and lowest values in mid-elevation tropical forests with dense understory vegetation and hence limited light availability. Higher diversity and biovolume of soil phototrophs were associated with less productive, well-illuminated soils with lower organic matter and nitrogen content and higher pH, phosphorus and cation content. Changes in microbial richness and biovolume across tropical forests showed a U-shaped elevation pattern, with higher values recorded in coastal and lowland forests up to 1000 m elevation, the lowest values in the mid-elevation open-canopy forests with dense understory vegetation caused by disturbances of forest elephants and higher values again in montane forests between 1800 and 2200 m. Above the tree line, soil phototrophic biovolume also showed a U-shaped elevation pattern, with lower richness recorded in compact grasslands between 2700 and 3400 m. At lower-elevation savanna, soil phototrophs are indirectly supported by regular fires during the dry season, which reduces plant cover and increases soil phosphorus and cations, while barren lava fields at higher elevations around the summit support soil phototrophs directly via increased soil P and K content and indirectly by inhibiting plant growth and vegetation cover. Our results shed light on an overlooked part of soil biodiversity in major tropical ecosystems and uncover the role of various ecological filters in structuring phototrophic microbial communities in tropical soils.