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.