Abstract
Understanding the processes that influence the structure of biotic communities is one of the major ecological topics, and both stochastic and deterministic processes are expected to be at work simultaneously in most communities. Here, we investigated the vertical distribution patterns of bacterial communities in a 10-m-long soil core taken within permafrost of the Qinghai-Tibet Plateau. To get a better understanding of the forces that govern these patterns, we examined the diversity and structure of bacterial communities, and the change in community composition along the vertical distance (spatial turnover) from both taxonomic and phylogenetic perspectives. Measures of taxonomic and phylogenetic beta diversity revealed that bacterial community composition changed continuously along the soil core, and showed a vertical distance-decay relationship. Multiple stepwise regression analysis suggested that bacterial alpha diversity and phylogenetic structure were strongly correlated with soil conductivity and pH but weakly correlated with depth. There was evidence that deterministic and stochastic processes collectively drived bacterial vertically-structured pattern. Bacterial communities in five soil horizons (two originated from the active layer and three from permafrost) of the permafrost core were phylogenetically random, indicator of stochastic processes. However, we found a stronger effect of deterministic processes related to soil pH, conductivity, and organic carbon content that were structuring the bacterial communities. We therefore conclude that the vertical distribution of bacterial communities was governed primarily by deterministic ecological selection, although stochastic processes were also at work. Furthermore, the strong impact of environmental conditions (for example, soil physicochemical parameters and seasonal freeze-thaw cycles) on these communities underlines the sensitivity of permafrost microorganisms to climate change and potentially subsequent permafrost thaw.
Highlights
Characterizing species diversity and its variation, or understanding the forces that structure ecological communities and their spatial patterns along environmental gradients is a central theme of ecological research, and both niche-related and neutral processes are generally thought to be important [1,2,3,4]
In recognition of the potential limitations of focusing solely on taxonomic beta diversity, community ecologists have recently extended this speciesbased metric to include a phylogenetic component of beta diversity which should permit the inferences of relative roles of deterministic and stochastic processes that are more directly connected to ecological, historical, and evolutionary processes [11,16,17,18,19]
Our soil samples were collected from different depth intervals within a 10-m-long permafrost core, which progresses from surface active layer soil into a layer of permafrost soil
Summary
Characterizing species diversity and its variation, or understanding the forces that structure ecological communities and their spatial patterns along environmental gradients is a central theme of ecological research, and both niche-related (deterministic) and neutral (stochastic) processes are generally thought to be important [1,2,3,4]. Taxonomic beta diversity ( referred to as taxonomic or species turnover), i.e., the change in community structure between sampling units along a spatial, temporal or environmental gradient [1], has provided important insights into the relative roles of deterministic and stochastic processes by relating the amount of turnover to variation in spatial distance and the abiotic environment [13,14,15]. In recognition of the potential limitations of focusing solely on taxonomic beta diversity, community ecologists have recently extended this speciesbased metric to include a phylogenetic component of beta diversity (phylogenetic beta diversity or phylogenetic turnover) which should permit the inferences of relative roles of deterministic and stochastic processes that are more directly connected to ecological, historical, and evolutionary processes [11,16,17,18,19]
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