Differential roles of deterministic and stochastic processes in structuring soil bacterial ecotypes across terrestrial ecosystems

Landfill depths alter microbial community structure and ecological assembly by affecting δ13C and organic matter

Microorganisms are crucial for the degradation of organic matter during landfill. However, the processes of microbial community assembly and ecological niche in landfill are poorly understood. Here, the mechanisms underlying microbial assembly in landfill were investigated based on neutral theory, niche distribution and network analysis. The results showed that moisture and potassium in the landfilled wastes increased with depth, while organic matter and δ13C were significantly higher in the middle layer than in the surface and bottom layers (P < 0.05). The richness and diversity of bacteria were significantly greater in the surface layer compared to the middle and bottom layers (P < 0.05), with moisture content, organic matter, total nitrogen and δ15N significantly influencing bacterial community composition. Deterministic processes over stochastic processes were pronounced in the surface layer, with the latter accounting only for 3.8 %. As landfill depth increased, variations in organic matter composition led to a greater influence of stochastic processes (52.7 %), while deterministic processes accounted for only 5.8 %. The niche breadth of abundant taxa was narrower than conditionally rare or abundant taxa, with their distribution primarily regulated by waste δ13C (P < 0.05), indicating greater environmental sensitivity. The niche overlap of microbial communities was lower in the surface layer, with the proportion of groups with high niche overlap being 2.69 times and 1.69 times higher in the middle and bottom layers. This study provided the first analysis of microbial niche dynamics across landfill depths, revealing critical interactions between δ13C driven organic matter availability and stochastic assembly processes.

Distinct Assembly Patterns of Soil Bacterial and Fungal Communities along Altitudinal Gradients in the Loess Plateau's Highest Mountain

A critical issue in microbial ecology is quantifying the relative contributions of deterministic and stochastic processes to microbial community assembly, and predicting ecosystem function by understanding the ecological processes of community composition is an integral part. However, the mechanisms driving microbial community assembly along altitudinal gradients in mountain ecosystems remain largely unexplored. Here, we used high-throughput sequencing to examine the structural characteristics and diversity maintenance mechanisms of soil bacterial and fungal communities along an altitudinal gradient (2632-3661 m) in Mahan Mountain, the highest peak of the Loess Plateau. Proteobacteria, Acidobacteriota and Actinobacteriota dominated the bacterial communities, while Ascomycota, Basidiomycota and Mortierellomycota were the predominant fungal groups. Although elevation did not significantly affect bacterial and fungal alpha diversity, notable shifts in community structure were observed along the altitudinal gradients. Bacterial communities were predominantly shaped by deterministic processes, leading to pronounced structural and compositional differentiation across altitudes. In contrast, fungal community assembly was primarily determined by a combination of deterministic and stochastic processes, leading to small pronounced structural divergence. The interplay of topography, climate, and soil conditions influenced the altitudinal distribution and community structure of soil bacteria in this mountain ecosystem.

Differential roles of deterministic and stochastic processes in structuring soil bacterial ecotypes across terrestrial ecosystems

Soil bacteria are vital to ecosystem resilience and resistance, yet ecological attributes and the drivers governing their composition and distribution, especially for taxa varying in ecological traits and inhabiting different ecosystems, are not fully understood. Here, we analyzed a large-scale bacterial community and environmental dataset of 622 soil samples systematically collected by us from six major terrestrial ecosystems across the United States. We show that soil bacterial diversity and composition significantly differ among ecotypes and ecosystems, partially determined by a few universal abiotic factors (e.g., soil pH, calcium, and aluminum) and several ecotype- or ecosystem-specific ecological drivers. Co-occurrence network analysis suggests that rare taxa have stronger ecological relevance to the community than abundant taxa. Ecological models revealed that deterministic processes shape assembly of abundant taxa and generalists, while stochastic processes played a greater role in rare taxa and specialists. Also, bacterial communities in the shrubland ecosystem appear to be more sensitive to environmental changes than other ecosystems, evidenced by the lowest diversity, least connected community network, and strongest local environmental selection driven by surrounding land use. Overall, this study reveals ecological mechanisms underlying the bacterial biogeography in terrestrial ecosystems nationwide and highlights the need to preserve rare biosphere and shrubland ecosystems amid environmental disturbance.