F.S. Chapin et al.’s popular textbook Principles of Terrestrial Ecosystem Ecology states that the parts of soil are “arranged in a relatively predictable vertical structure” (p. 58 in the 2002 edition). The theoretical patterns for vertical soil structure (e.g., A-B-C ordering of horizons) are a basis for research methods and our understanding of ecosystem structure and function in general. A general understanding of how urban soils differ from non-urban soils vertically is needed to inform urban research methods and advance our knowledge of urban ecosystems. We performed a soil taxonomic assessment of 391 deep soil cores (up to 5-m) collected in 12 cities across the U.S., Puerto Rico, and the Republic of the Marshall Islands covering 10 soil orders. The likely soil series in the absence of urbanization was identified for each soil core and comparison soil taxonomy data for agricultural and wildlands land uses soils of the same soil series was gathered from the NASIS database. We hypothesized that urbanization has modified the vertical arrangement of soil horizons and decreased their vertical complexity (e.g., degree of horizonation). In addition, we hypothesized that these modifications would be greatest near the surface, decreasing with depth.
Preliminary analysis using data from only two of the cities (Detroit and Cleveland) indicated that urban soils had fewer soil horizons than their non-urban references. The order of horizons was also different from reference and theoretical soil structure. Namely A-C transitions with an absence of B horizons was more common in urban than non-urban soils. B horizons were often absent or their first occurrence was as a substratum of the C horizon. The simplification of B horizonation or loss of B horizons accounted for a major proportion of the decline in the degree of horizonation in urban relative to non-urban soils. If these patterns hold across more cities, our results indicate a need to revisit the conventional predictions of vertical soil structure for understanding urban ecosystems. By identifying the patterns that are found across cities, urban soil vertical structure may still be relatively predictable like their non-urban counterparts.