Abstract
The Northern high latitudes are warming twice as fast as the global average, and permafrost has become vulnerable to thaw. Changes to the environment during thaw leads to shifts in microbial communities and their associated functions, such as greenhouse gas emissions. Understanding the ecological processes that structure the identity and abundance (i.e., assembly) of pre- and post-thaw communities may improve predictions of the functional outcomes of permafrost thaw. We characterized microbial community assembly during permafrost thaw using in situ observations and a laboratory incubation of soils from the Storflaket Mire in Abisko, Sweden, where permafrost thaw has occurred over the past decade. In situ observations indicated that bacterial community assembly was driven by randomness (i.e., stochastic processes) immediately after thaw with drift and dispersal limitation being the dominant processes. As post-thaw succession progressed, environmentally driven (i.e., deterministic) processes became increasingly important in structuring microbial communities where homogenizing selection was the only process structuring upper active layer soils. Furthermore, laboratory-induced thaw reflected assembly dynamics immediately after thaw indicated by an increase in drift, but did not capture the long-term effects of permafrost thaw on microbial community dynamics. Our results did not reflect a link between assembly dynamics and carbon emissions, likely because respiration is the product of many processes in microbial communities. Identification of dominant microbial community assembly processes has the potential to improve our understanding of the ecological impact of permafrost thaw and the permafrost–climate feedback.
Highlights
Permafrost, soil that has been frozen for two or more consecutive years, underlies approximately one fourth of the northern hemisphere (Zhang et al, 1999) and is undergoing thaw with increasing global temperature (Romanovsky et al, 2017)
The soil C:N ratio was approximately four times higher in the upper active layer compared to the permafrost and consistently decreased with depth (Figure 2; analysis of variance (ANOVA); F = 7.854, P = 0.0000574)
Our findings suggest there is a shift toward stochastic assembly immediately after thaw, the influence of drift, except in upper active layer soils
Summary
Permafrost, soil that has been frozen for two or more consecutive years, underlies approximately one fourth of the northern hemisphere (Zhang et al, 1999) and is undergoing thaw with increasing global temperature (Romanovsky et al, 2017). Soil microorganisms decompose this carbon resulting in the release of greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere (Schädel et al, 2014; Treat et al, 2016; Voigt et al, 2017). As a result, these gases create a positive feedback to global warming, further threatening permafrost degradation. Frozen conditions promote genes involved in stress responses and survival strategies, and thaw results in increases in genes involved in decomposition of soil organic matter and transport of soil nutrients (Mackelprang et al, 2011, 2017; Coolen and Orsi, 2015; Hultman et al, 2015)
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