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.