Interactions of Microhabitat and Time Control Grassland Bacterial and Fungal Composition

Abstract

Dryland grasslands are vast and globally important and, as in all terrestrial ecosystems, soil microbial communities play fundamental roles in regulating dryland ecosystem function. A typical characteristic of drylands is the spatial mosaic of vascular plant cover surrounded by interspace soils, where biological soil crusts (biocrusts)-a complex community of organisms including bacteria, fungi, algae, mosses, and lichens-are common. The implications of this heterogeneity, where plants and biocrust cover co-occur, are often explored in the context of soil fertility and hydrology, but rarely has the impact of these multiple microhabitat types been simultaneously explored to determine the influence on bacterial and fungal communities, key biological players in these ecosystems. Further, our understanding of the temporal dynamics of bacterial and fungal communities in grasslands, and of how these dynamics depend on the microhabitat within the ecosystem, is notably poor. Here we used a temporally and spatially explicit approach to assess bacterial and fungal communities in a grassland on the Colorado Plateau, and to link variation in these communities to edaphic characteristics. We found that microhabitat (e.g., vascular plant rhizosphere, biocrust, and below biocrust) was the strongest driver of differences in bacterial and fungal community richness, diversity, and composition. Microhabitat type also significantly mediated the impact of temporal change in shaping community composition. Taken together, 29% of the variation in bacterial community composition could be explained by microhabitat, date, and microhabitat-by-date interactions, while only 11% of the variation in fungal community composition could be explained by the same factors, suggesting important differences in community assembly processes. Soil microbial communities dictate myriad critical ecosystem functions, thus understanding the factors that control their compostition is crucial to considering and forecasting how terrestrial ecosystems work. Overall, this case study provides insights for future studies on the spatial and temporal dynamics of bacterial and fungal communities in dryland grasslands.

Stochastic Dispersal Rather Than Deterministic Selection Explains the Spatio-Temporal Distribution of Soil Bacteria in a Temperate Grassland

“…In general, soils are regarded as the habitat harboring the most temporally invariant microbial communities ( Shade et al, 2013 ), likely resulting from the many micro-habitats present in soils at very small scales ( Tecon and Or, 2017 ), which in theory should limit the rate and success of independent dispersal processes (e.g., Yan et al, 2019 ). However, reports of soil microbial communities with pronounced temporal variability related to season, soil, and management type are accumulating ( Lauber et al, 2013 ; López-Mondéjar et al, 2015 ; Zifcakova et al, 2016 ; Albright et al, 2019b ; Landesman et al, 2019 ). In our case, the sampling season significantly structured α- and – to a lesser degree – β-diversity and resulted in specific abundance patterns across all taxonomic levels.…”

“…Taken together, the detected assembly principles at our site allow the interpretation that an apparent absence of physical barriers in the soil resulted in homogenization of an intact community that had been established long before the experiment. Interestingly, a similar experiment with dryland soils featuring a mosaic of crusts and plant cover contrastingly showed high β-diversity on very small scales, driven by habitat turnover (Albright et al, 2019b).…”

“…There is a rich body of studies covering either spatial or temporal variability of soil microbiomes, but investigations of both dimensions simultaneously are rare, especially at the plot scale. Studies have often explicitly selected sites with strong habitat and/or seasonal turnover on small scales and found effects correlating with this high environmental variability present in their sites (Mummey and Stahl, 2003;Henneberger et al, 2015;Hill et al, 2015;Kivlin and Hawkes, 2016;Albright et al, 2019b). Importantly, none of these study designs were used to study assembly processes of bacteria.…”

Biological Soil Crust From Mesic Forests Promote a Specific Bacteria Community

“…Archaea, Chloroflexi, and anaerobic bacteria appeared to be less prominent in the biocrusts than below the biocrust ( Steven et al, 2013 ). Data on the bacterial community in biocrusts and underlying soils of the same sampling site are so far restricted to semiarid sites ( Steven et al, 2013 ; Albright et al, 2019 ; Moreira-Grez et al, 2019 ; Pombubpa et al, 2020 ). These studies at different arid geographical regions, Western Australia, Colorado Plateau, and the Mojave Desert, United States, found major differences in the bacterial communities between biocrusts and underlying soils.…”

Microbial Dispersal, Including Bison Dung Vectored Dispersal, Increases Soil Microbial Diversity in a Grassland Ecosystem

“…Contrary to our predictions, however, landscape-scale bison grazing and fire management treatments did not mediate microbial dispersal effects ( Figure 4 and Table 3 ). The lack of dispersal differences could be because very local scale effects, such as soil openness to dispersal ( Albright et al, 2019 ) and influx of microbial populations from neighboring (sub-centimeters) soil and dung, might matter more than watershed scale environmental factors for overall dispersal rates. Alternatively, the effects of fire and grazing on dispersal might shift with time and our 3-month experiment may not have been long enough to capture this temporal variation.…”