Ecosystem functioning of urban soils: A global perspective
Urban land-use change (LUC) can affect soil characteristics and associated biogeochemical cycles through altered disturbance regimes, management practices (e.g., irrigation), built structures, and modified environments (e.g., heat island). As a result, the conversion of native to urban ecological systems should significantly affect the role soil plays in biogeochemical cycles at multiple scales. The net effect of urban LUC on soil biogeochemical cycles will vary depending on the pre-existing ecosystem and the human response to characteristics of that ecosystem. For instance, development patterns and the need for infrastructure are strongly affected by topography and site limitations (e.g., soil drainage), while human decisions such as cultivating non-indigenous vegetation (e.g., turfgrass) results in management inputs driven by regional edaphic and climate conditions. In addition, human preferences for urban vegetation are strongly related to cultural factors. Therefore, the effect of urban LUC on soil biogeochemical cycles should vary by ecoregion and also be coupled to land management, land cover, and cultural preferences. Our objectives are to 1) assess the net effect of urban LUC on soil biogeochemical cycles at multiple scales and 2) in light of these effects, discuss the potential for urban soils to provide ecosystem services.
Urban soils are generally expected to be highly disturbed and heterogeneous with little systematic pattern in their characteristics. However, our observations of urban soils at multiple scales have shown that the chemical, physical, and biological response of soils to urban LUC exhibit patterns in time and space. For example, the bulk density of surface soils can explain up to 50% of the variability of organic matter concentration for an urban area. Indeed, our analysis suggests that urban soils are often undisturbed, of high fertility and function, especially when not sealed by impervious surfaces. Moreover, when compared to their native counterparts, unsealed urban soils can store as much or more carbon per unit area, e.g., 10.8 vs. 7.7 kg m-2 in the Atlanta region, respectively. Finally, a global comparison of urban soils in 5 cities across different biomes shows that soils resulting from a disturbance or managed as turfgrass exhibited similar chemical, physical, and biological characteristics, while undisturbed soils occurring in the same cities varied more widely. In conclusion, urban effects on soil biogeochemical cycles occur at multiple scales, with a wide variance exhibited within a metropolitan area but with relatively low variance across urban areas at continental and global scales.