Introduction
Urbanization has drastic impacts on geochemistry, climate, and biota,
including diverse microbiomes. Although urban areas occupy less than
0.5% of global land area (Scheider et al. , 2009), urban land
cover continues to expand, which could have substantial consequences for
environmental health and sustainability (Seto et al. , 2012).
Urbanization causes landscape fragmentation, which can reduce plant and
animal biodiversity (Su et al. , 2012; Liang et al. , 2008;
Delaney et al. , 2010). Urban light and sound pollution can alter
animal behavior, disrupt species interactions, and cause shifts in
species richness and composition (Longcore & Rich, 2004; Ciach &
Fröhlich, 2017; Firebaugh & Haynes, 2016; Francis et al. , 2009).
Soils in cities are often contaminated with organic pollutants and heavy
metals. These contaminants can stress plants, contaminate plant tissues,
impact soil and pollinator animal communities, and pose health risks for
human residents (Fryzova et al. , 2017; Hernandez & Pastor, 2008;
Pavao-Zuckerman & Coleman, 2007; Tauqueer et al. , 2013; Panet al. , 2018; Wang et al. , 2013). The environmental impact
of urban land use can reach far beyond city limits through greenhouse
gas emissions (Pichler et al. , 2017), atmospheric nitrogen
deposition (Fenn et al. , 2003), and water pollution (Russelet al. , 2008; Overbo et al. , 2021; Wright et al. ,
2010).
At the same time, urban environments sustain critical ecosystem
processes. For example, sprawling urban areas continue to provide
sufficient habitat, resources, and dispersal routes to support a high
level of biodiversity (Wenzel et al. , 2020; Angold et al. ,
2006). Insect pollinators can thrive in urban landscapes, which has made
them a focus of urban conservation efforts (Baldock et al. , 2019;
Hall et al. , 2016). Urban green spaces can help to offset impacts
of urbanization by filtering air, regulating climate, and slowing runoff
(Bolund & Hunhammar, 1999; McPhearson et al. , 2015). Urban soils
support nutrient cycling processes and, with proper management, may be
effective at sequestering carbon (Pouyat et al. , 2010; Brownet al. , 2011). While urban landscapes appear quite different from
their natural counterparts, cities continue to support diverse and
functional ecosystems. Understanding these novel urban ecosystems can
help inform management strategies and maintain vital ecosystem processes
that make cities more sustainable.
In addition to flora and fauna, soil microorganisms are essential for
ecosystem functioning and can provide ecosystem services. However, soil
microbial communities have been largely overlooked in urban ecology
research. Only recently has there been a push to understand the impact
of urbanization on the soil microbiome (Antwis et al. , 2017).
This is a rapidly emerging area of research and, to our knowledge, there
is not yet an overarching conceptual framework for effectively
developing and answering critical questions about urban soil microbial
communities.
In this paper we propose a new framework to advance research on urban
soil microbial communities and their role in ecosystems. This framework
combines and expands on previous advances in urban soils, urban ecology,
and microbial ecology. We apply our framework to synthesize previous
findings and discuss the implications of urban soil microbes for
ecosystem and human health. We find that, strikingly, there has been
very little work done to link microbial taxa to functioning in urban
soils – information which could guide urban sustainability efforts and
our fundamental understanding of microbial structure-function
relationships. Finally, we offer recommendations for research priorities
and practices to guide the field of urban microbial ecology in answering
these crucial questions. We emphasize the need for collaboration between
ecologists, biogeochemists, and social scientists to gain a holistic
understanding of microbes and their interactions with humans in the
urban environment.