Microbial community structure across grazing treatments and environmental gradients in the Serengeti

Field-based observational research is the first step in understanding the
factors that structure microbial communities and generate biogeography of
soil microbes. As one of the last remaining naturally grazed ecosystems on
Earth, the Serengeti National Park in Tanzania is an ideal location to
study the influence of large migratory mammals on microbial communities.
Also, active volcanoes generate strong environmental gradients due to ash
deposition and a rain shadow. We used 16S rRNA amplicons to characterize
bacterial and archaeal communities in soils from a 13-year herbivore
removal experiment to study the influence of grazing and environmental
gradients on the natural distribution of soil microbes. Removal of
mammalian herbivores shifted microbial community structure, with 31
amplicon sequence variants (ASVs) that were significant indicator taxa of
the ungrazed treatment and only three ASVs that were significant
indicators of the grazed treatment. The abundance of many ASVs were
correlated with soil texture, phosphorus, iron, calcium and rainfall, and
the evenness of taxa within samples increased with fine-textured soil.
Bayesian general linear mixed effects  models that parse the
relative importance of multiple, highly correlated predictors of beta
diversity were consistent with a significant, but weak (2%), effect of
grazing, and stronger effects of phosphorus (14%) and silt (14%) contents
in soil. Beta diversity of microbial communities was greater in grazed
than in ungrazed plots; consequently, our results suggest that the impacts
of grazing on the community assembly of microbes results from
deterministic environmental filtering caused by the influence of
herbivores on plant communities and soil properties rather than stochastic
dispersal via herds of large mammalian herbivores. These herbivore effects
are superimposed on deterministic environmental filtering by natural soil
and precipitation gradients across the Serengeti.