Stressful, isolated, yet diverse: Green roofs have rich microbiomes that are not dominated by oligotrophic taxa.

Green roofs are unique ecosystems combining two major community assembly filters, namely stress and spatial isolation. As such, they represent an interesting model ecosystem in community ecology. In this study, we characterized the microbiome structure on 19 green roofs and 5 urban parks as a benchmark comparison (i.e. non-isolated, non-stressful habitats). Green roofs were not species depauperate, showing similar α-diversity compared to surrounding parks. We also did not find an overrepresentation of bacterial phyla typically recognized as oligotrophs, which calls into question the notion of green roofs as highly stressful habitats for bacteria, and/or the conservatism of nutritional ecophysiology at the phylum level. The geographical position of a roof, or its degree of spatial isolation (assessed through its height and area) were not important predictors of microbiome diversity and structure, suggesting that dispersal limitations impose little constraints on green roof microbiome assembly. Finally, key microbial groups (e.g. archaeal nitrifiers, Actinobacteria) were much less frequent and/or abundant on green roofs, which may have important implications for nutrient cycling and urban biogeochemistry. More work will be required to phenotype the microorganisms overrepresented on green roofs and specifically measure key soil processes in these unique urban ecosystems.