Soil properties and road proximity likely influence plant and bacterial traits and diversity more than habitat size in small urban greenspaces

Societal impact statement The global decline in biodiversity can be partly ameliorated by sympathetic design of green spaces within urban environments. This includes roundabouts on road systems. This study utilised a combination of approaches to assess levels of plant and bacterial diversity in roundabouts of varying size, soil characteristics and habitat complexity. Larger roundabouts typically had more varied soil characteristics, which in turn increased plant and microbial diversity. Urban planners should integrate large roundabouts into developments and utilise the space within the roundabout to incorporate a variety of soil types to maximise community development and hence biodiversity. Summary Legislation requires that wildlife habitat is incorporated into urban areas to tackle the global biodiversity crisis. Crucial to the success of urban habitat is their design. Unlike other urban habitats, such as green roofs, roundabouts are understudied with little information available to local councils on how to design roundabouts with biodiversity in mind. We investigated the impact roundabout size, soil properties and distance from the road has upon plant and bacterial biodiversity. We used vegetation surveys, meta‐barcoding and analysis of traits and strategies to understand how biodiversity changes with roundabout size and how communities respond to roundabout features. We found that plant and bacteria richness does not change with roundabout size, instead soil moisture shaped trends in diversity although also increasing with roundabout area. Plant and bacteria communities showed varying responses to soil properties and road proximity. Results may enable local councils to design roundabouts with plant and bacteria biodiversity in mind, providing useful opportunities for urban intervention within the context of the biodiversity crisis. Our key advice to local councils is to prioritise larger roundabouts with high‐quality soil environments, which can support diverse communities nontypical of urban habitats.