Global comparison shows that soil bacterial communities in extreme pH soils are more structured by deterministic processes.

A major aim of microbial ecology is the search for basic 'rules' that dominate variation in microbial communities. An earlier comparison of several soil successional series showed that pH explained variation in the relative importance of stochastic versus deterministic processes in bacterial communities. In neutral pH soils, bacterial communities were more strongly influenced by stochastic processes than in low or high pH soils. Here, we took a broad level approach to attempt a more definitive answer of whether soil pH dominates bacterial community structuring using the global database of 237 samples. The beta-NTI showed that at both a global and continental scale, samples with low pH were dominated by deterministic processes, while in samples at around neutral pH, stochastic processes dominated. At high pH, stochasticity dominated on the global scale, but on several continents, the beta-NTI showed determinism predominating. Overall, it appears that bacterial community structuring is strongly and predictably affected by pH, with the most consistent difference observed between determinism at low pH and stochasticity at neutral pH. There is a need for hypothesis testing to explain why this trend exists. It is possible that at low pH, there is a greater selection for consortia to exploit resources, which leads to more predictable, deterministic combinations of species co-occurring. Additionally, the high energy demands for homeostasis and the constraints from the lack of available nutrient resources may impose greater niche-based competition, resulting in more deterministic community structuring at low pH.

Responses of soil micro-eukaryotic communities to decadal drainage in a Siberian wet tussock tundra.

Climate warming holds the potential to cause extensive drying of wetlands in the Arctic, but the warming-drying effects on belowground ecosystems, particularly micro-eukaryotes, remain poorly understood. We investigated the responses of soil micro-eukaryotic communities, including fungi, protists, and microbial metazoa, to decadal drainage manipulation in a Siberian wet tundra using both amplicon and shotgun metagenomic sequencing. Our results indicate that drainage treatment increased the abundance of both fungal and non-fungal micro-eukaryotic communities, with key groups such as Ascomycota (mostly order Helotiales), Nematoda, and Tardigrada being notably abundant in drained sites. Functional traits analysis showed an increase in litter saprotrophic fungi and protistan consumers, indicating their increased activities in drained sites. The effects of drainage were more pronounced in the surface soil layer than the deeper layer, as soils dry and warm from the surface. Marked compositional shifts were observed for both communities, with fungal communities being more strongly influenced by drainage-induced vegetation change than the lowered water table itself, while the vegetation effect on non-fungal micro-eukaryotes was moderate. These findings provide insights into how belowground micro-eukaryotic communities respond to the widespread drying of wetlands in the Arctic and improve our predictive understanding of future ecosystem changes.