Sealing impacts soil properties and the soil microbial community in urban areas

Urbanisation inevitably alters the chemical, physical and biological soil properties. Consequently, it modifies environmental soil services. The sealing of soil – covering the soil surface with non-biological structures - has gained attention in urban area in relation to flooding, loss of soil carbon, loss of soil function and other problems, which can be mitigated introducing greenspaces in urban areas. To understand the impact of sealing and greenspaces on the urban soil properties, soil beneath three pavement types (concrete, slab and tarmac) and from paired unsealed areas (representing four cover types – bare soil, grass, shrub and tree) were sampled in forty sites in Lancaster, UK to access chemical [pH, total carbon (C), total nitrogen (N), C/N ratio], physical [moisture] and microbiological [microbial biomass (MB), 16S rDNA metabarcoding data metrics] parameters. Sealed soils had lower moisture, C, N and MB, and higher pH than unsealed soils. C/N ratio had no significative difference. Changes in chemical and physical soil parameter affected the bacterial diversity indexes (richness, Shannon, Simpson and α parameter of Fisher’s log series), which were significantly lower in sealed areas as well. NMDS analysis showed high variability in the bacterial community of sealed soil, but lower variability in unsealed soil. The sum of sequences of gram positive, gram negative, oligotrophic and copiotrophic bacteria, as well gram positive:gram negative  ratio (GP:GN), were lower in sealed soil. The reduced GP:GN ratio on sealed soil suggests low-quality organic matter and the impact of low moisture and N on bacterial community. Pavement type had no effect on sealed soil parameters. However, the cover type significantly changed GP:GN and copiotrophic:oligotrophic ratio on unsealed soil, indicating that soil in grassland may have higher resource availability (C and N) and microbial growth rates than shrub and trees. Our results reinforce that the sealing, unrelated to pavement type, has a negative impact on soil properties and deplete microbial diversity. Cover type may affect resources availability on unsealed soil. These changes can affect microbial processes related to biogeochemical cycles, impacting the carbon store and the potential for nutrient cycling in urban soils.