Soils in urban and urbanizing landscapes are profoundly modified resulting in altered composition of the biota. However, in cities, ecosystem services generally provided by soils are just as essential for the functioning of the aboveground biota as they are in more natural systems. Soils can function in urban landscapes by reducing the bioavailability of pollutants, storing carbon and mineral nutrients, serving as habitat for soil fauna, and moderating the hydrologic cycle. The physico-chemical and biological processes of the soil systems are tightly coupled: while soil abiotic factors determine the occurrence and abundance of soil biota, organisms are active participants in soil biogeochemical cycles and thus the ecosystem services mentioned above. Urban soil fauna and the urban soil food web are altered due to 1) local extinction of native species 2) high percentage of non-native species, and 3) highly modified base of the decomposer food web. Moreover, soil community structure is highly dependent on the level of land management. In this presentation I first will discuss a conceptual framework for understanding community assembly in urban systems. I will then provide examples on the effects of altered species composition on ecosystem function, focusing on studies conducted in the Baltimore Ecosystem Study LTER, MD. Examples will include the role of earthworms in nitrogen cycling and fate of pollutants, and the effects of forest fragmentation on decomposer beetle communities. Knowledge of soil biota and their functions is essential to restore and maintain the biological functions in urban soils.