Microbial biodiveristy in urban soils
With ongoing population growth and large-scale urbanization, urban ecosystems are now the predominant habitats for humans. Yet, the biodiversity and dynamics of these ecosystems have been studied only sparsely, especially considering soil microbes. Soils in cities are disturbed and largely covered by (semi-)impervious covering, except in the highly fragmentized urban greenspaces. Street trees are one of the smallest and often most confined greenspaces and can serve as a model system to better understand the urban soil microbiome. We used DNA metabarcoding to characterize bacterial and fungal communities in five urban typologies: 1) open soils around street trees (treepits) (Tilia spp.) and soils underneath sidewalks and roadways 2-3) with and 4-5) without these trees nearby; in five cities in The Netherlands (Amsterdam, Den Haag, Dordrecht, Leiden, Tilburg). Bacterial alpha diversity was consistent in all typologies, while fungal richness was 64% higher with street trees nearby and was 170% higher in open treepits, when compared to typologies without trees. Bacterial and fungal community structure were influenced by soil covering as well as presence of trees consistently throughout the cities. Treepits harboured more known nitrogen-cycling bacteria and nearly all fungal traits were overrepresented in the presence of trees, indicating more functional redundancy. Community assembly differed between typologies: bacterial communities were shaped by homogeneous selection which correlates with sensitivity to edaphic factors, whereas fungal assembly was dominated by dispersal limitation in covered areas, but not in the treepits. Taken together, street trees act as hotspots for microbial soil biodiversity extending even underneath paving.