Urban soils represent a wide range of conditions from remnant patches retaining original horizonation and structure to newly constructed, engineered soils. Contrary to the general view of urban soil as highly disturbed, polluted, and lifeless medium, a surprisingly rich belowground community exists in cities. Addressing soil biota responses to urbanization is particularly challenging due to their large size range, high functional, taxonomical and phylogenetic diversity, and cryptic nature.
Our goal is to provide a framework to investigate the urban soil landscape and its biota. We begin by linking existing frameworks that explain direct and indirect influences of urbanization on soils and the production of spatial heterogeneity in cities through different management practices. Using this framework, we will present examples of the responses of the soil decomposer community to drivers often associated with urbanization. We will review existing literature addressing the adaptations and responses of urban soil decomposer communities, and supplement this review with data from the Baltimore Ecosystem Study and the recently established Global Urban Soil Ecology and Education Network (GLUSEEN).
Very few in situstudies exist that address the complex behavioral, physiological, functional trait responses of soil organisms to urbanization, and none at the genetic level, although some address community-scale responses. Some conclusions can be drawn from laboratory studies, and field studies from similar environments. For instance, metal pollution is undoubtedly a stress factor, and soil invertebrates exhibit a variety of responses to elevated levels of Pb, Zn, Cd, Cu. Isopods and earthworms avoid polluted food resources, and eliminate or store heavy metals in specialized organs. Both taxa exhibit huge individual variation with regard to physiological responses, which indicates the potential to develop pollutant tolerance.
Conditions are not always stressful for in urban soil. Irrigation and organic matter amendment can enhance both diversity and abundance of soil organisms, but also can shift community composition. Novel resources in urban soils also emerge, such as pollutants (i.e., polycyclic aromatic hydrocarbons) and fast-food rubbish that support populations of soil microbes and invertebrates. More extreme adaptations to the built environment include living in a soil-less habitat for instance earthworms in sewer pipes.
Our review identifies significant gaps and opportunities to understand the adaptations and responses of urban soil decomposer communities. Future studies should clearly connect a specific environmental driver/selective force to a given population and examine its effect on the appropriate spatial and temporal scale, and organizational level.