Urban soils may be higher quality than you think

One common assumption about urban ecosystems is that they are lower quality than their more rural counterparts. In some cases, this is true; species assemblages are generally less diverse in urban areas, while ecosystem functions are often relatively impaired. However, this is not always the case, as was recently shown in a study on soil compaction in the UK city of Leicester.

Soils are literally and metaphorically the foundation of most terrestrial ecosystem services, playing an integral role in nutrient and carbon storage, purification and drainage water, and provision of anchorage and physical support to both plants and buildings. The ability of soil to perform these functions is reduced by the process of compaction, which decreases the space between individual grains of dirt and makes it harder for plant roots and water to penetrate the earth. Compaction can be caused by heavy machinery and foot traffic--two things that can be found in abundance in urban areas.

Collaborators from the University of Sheffield, the University of Kent, and the NERC Centre for Ecology and Hydrology joined forces to measure soil compaction at 136 sites within Leicester and 28 agricultural sites around the city's border. The sites included gardens, non-domestic land, pasture, and arable fields; at each, soil samples were taken from the top 0-7 cm of earth and also at 7-14 cm below the surface. Bulk density--a measure of soil compaction--was measured in each sample. In addition to investigating whether bulk density differed at each depth over the urban and rural sites, the researchers also looked to see whether it was significantly associated with land cover type (including lawn, woody vegetation, herbaceous growth, shrubs, and trees) at each sample site.

At the 0-7 cm depth, bulk density of urban soils varied much more widely than that of agricultural soils, but overall had a significantly smaller mean value. In other words, agricultural soils, rather than urban soils, were consistently more compacted. At the 7-14 cm depth, there was again more variation at urban sites, but average bulk density did not differ much between the agricultural and urban areas. Across both types of site, soil compaction was significantly influenced by land cover: Bulk density at arable sites was higher than in urban sites with woody garden, tree and shrub, and tall shrub land cover. The least amount of soil compaction was found in green spaces dominated by woody vegetation (e.g., trees and shrubs).

Bulk density values measured within Leicester and its surrounding agricultural fields are comparable to those reported for similar sites elsewhere in Europe, as well as in the United States and Asia. Thus, worldwide, urban soils are in pretty good shape--in comparison with agricultural fields, anyway, where decades of management with heavy machines and inputs have led to much higher levels of soil compaction in the top 7 cm of earth. Another encouraging sign is that the highest bulk density recorded in urban sites was below the "cutoff" after which root growth is halved. 

Thus, it appears that urban sites are more than adequate for providing habitat for plant growth and reducing flooding by allowing storm water drainage. Therefore, it looks like we need to rethink our prejudiced opinions about urban habitats (in the case of soil compaction, anyway). Further, it might be time to think about what can be done to improve soil quality at agricultural sites--not only to increase output but also to ensure that farm habitats are capable of performing other valuable ecosystem services.

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Edmondson, J.L., Davies, Z.G., McCormack, S.A., Gaston, K.J., Leake, J.R. 2011. Are soils in urban ecosystems compacted? A citywide analysis. Biology Letters 7:771-774.

Thanks to the following websites for providing the images used in this post:
http://www.shelledwarriorsshop.co.uk/seed-growing-soil-with-added-calcium--free-post-1748-p.asp

http://www.developing8.org/2008/07/