Heterogeneity
At first glance, cities may appear to be a homogenous sea of concrete. However, the urban environment is composed of a highly diverse array of land-use types, ranging from parks and lawns dominated by turfgrass, to busy commercial centers with a mix of concrete and greenery, to large industrial complexes mainly characterized by impervious surfaces and polluted soils. These land use patches tend not to exist along a clear gradient, but are instead jumbled together to create a complex habitat mosaic (Figure 3) (Zhou et al. , 2018). Along with variation in land-use types, there is also heterogeneity of climate within urban spaces. Overall, cities tend to be hotter than their surrounding environment, a phenomenon known as the Urban Heat Island (e.g. Oke, 1995; Imhoff et al. , 2010; Li et al. , 2017). Within this heat island, a variety of micro-climates exist due to the position and size of buildings, density of trees and other green infrastructure, and other factors (Liao & Heo, 2018; Pincebourde et al. , 2016). Soils within a city can be trucked in from multiple non-local sources, and can vary in nutrient load, irrigation, heavy metal and pesticide pollution, and other characteristics depending on the management and development history of that land (De Kimpe & Morel, 2000; Zhiyanskiet al. , 2017; Karim et al. , 2014; Ziter & Turner, 2018).
How does the heterogeneity of urban habitats impact soil microbial community assembly, dispersal, and function? Understanding the role of landscape heterogeneity for microbial communities has only recently become a priority in microbial ecology as a whole. There is evidence that microbial communities vary with habitat heterogeneity (Horner-Devine et al. , 2004). However, due to microorganisms’ small size, their dispersal and survival may be constrained by different factors from macro-organisms (Martiny et al. , 2006) and therefore microbial response to habitat heterogeneity and patchiness, and the distance between patches, may not be predictable using our current theoretical frameworks based on macro-organism studies (Mony et al. , 2020). We do not know how the size of and distance between habitat patches in cities impact microbial communities, which should be a priority for future studies. However, there is some research suggesting that different urban land use types such as bioswales, parks, green roofs, and residential soils differ in microbial composition and diversity (Gill et al. , 2020; Wang et al. , 2018). Microbial litter decomposition also differs between urban soil types, indicating that microbial function may be affected by habitat type (Vauramo & Setala, 2011). Heterogeneity likely has an impact on the assembly and function of urban microbial communities, and future studies should investigate how microbial communities respond to patch type, size, edginess, and distance between patches.
While cities may be highly heterogenous at small to medium scales, it is possible that cities reduce environmental variation at regional and global scales. The “Urban Convergence” hypothesis states that urban areas are more similar to each other than to their surrounding rural environments, and some studies have found evidence for this trend with biological, geochemical, soil, and microclimate variables (Kaye et al. , 2006; Hall et al. , 2016; Herrmann et al. , 2020; Polsky et al. , 2014; McKinney, 2006; Groffman et al. , 2017; Pearse et al., 2016). However, no studies to our knowledge have investigated whether soil microbial communities converge in taxonomic identity or functioning across cities and, if so, what are the implications for ecosystem function. With a high degree of heterogeneity at neighborhood and city scales, and possible homogenization occurring at regional and global scales, it will be important to analyze urban soil microbial function at all of these scales.