TECHNICAL ARTICLE A Global Comparison of Surface Soil Characteristics Across Five Cities: A Test of the Urban Ecosystem Convergence Hypothesis Richard V.Pouyat,1 Ian D. Yesilonis,2 Miklós Dombos,3 Katalin Szlavecz,4 Heikki Setälä,5 Sarel Cilliers,6 Erzsébet Hornung,7 D. Johan Kotze,5 and Stephanie Yarwood8 Key Words: Anthropogenic soils, experimental network, soil carbon, Abstract: As part of the Global Urban Soil Ecology and Education urban soils Network and to test the urban ecosystem convergence hypothesis,were- port on soil pH, organic carbon (OC), total nitrogen (TN), phosphorus (Soil Sci 2015;180: 136–145) (P), and potassium (K) measured in four soil habitat types (turfgrass, ruderal, remnant, and reference) in five metropolitan areas (Baltimore, Budapest, rban soils provide many of the same ecosystem functions as Helsinki, Lahti, Potchefstroom) across four biomes. We expected the urban U “natural” and agricultural soils, for example, decomposition soil characteristics to “converge” in comparison to the reference soils. and nutrient cycling, water purification and regulation, and habitat Moreover, we expected cities in biomes with more limiting climatic condi- for an enormous diversity of organisms (Giller, 1996; Pouyat tions, or where local factors strongly affect soil characteristics, would ex- et al., 2010). Nonetheless, the increasing burden of expanding hibit the greatest variance across soil types within and among cities. In urban areas, concomitant need for new infrastructure, and accom- addition, soil characteristics related to biogenic factors (OC, TN) would modation of greater human population densities is resulting in a vary the most because of differences in climate and human efforts to over- worldwide alteration of these functions (Sachs, 2015). Therefore, come limiting environmental conditions. The comparison of soils among understanding the characteristics of urban soils and how they vary and within the five cities suggests that anthropogenic, and to a lesser degree is crucial to their management and restoration and ultimately to native, factors interact in the development of soils in urban landscapes. In the design of more sustainable cities (Pavao-Zuckerman, 2012; particular, characteristics affected by anthropogenic processes and closely Setälä et al., 2014). associated with biogenic processes (OC, TN) converged, while characteris- A central principle in urban ecological theory presupposes tics closely associated with parent material (K, P) did not converge, but that anthropogenic drivers dominate natural drivers in the control rather diverged, across all soil habitat types. These results partially sup- of ecosystem processes (Alberti, 1999; Kaye et al., 2006). If this ported the urban ecosystem convergence hypothesis in that a convergence assumption is true, it follows that both at regional and global occurred for soil characteristics affected by climatic conditions. However, scales ecosystem responses to urban land-use change should the divergence of K and P was unexpected and warrants adjusting the hy- converge relative to the native systems being replaced. This “con- pothesis to account for variations in anthropogenic effects (e.g., manage- vergence” in ecosystem properties is referred to as the urban eco- ment) that may occur within soil habitat types impacted by humans. system convergence hypothesis (Pouyat et al., 2003; McKinney, 2006; Pickett et al., 2008). In the case of the soil system, the con- 1U.S. Department of Agriculture Forest Service, Research & Development, vergence hypothesis suggests that soil responses will converge Washington, DC. across regional and global scales as long as anthropogenic drivers 2U.S. Department of Agriculture Forest Service, c/o Baltimore Ecosystem (e.g., management) dominate over natural soil-forming factors Study, Baltimore, MD. (e.g., relief). This convergence occurs because human effects on 3Institute for Soil Sciences and Agricultural Chemistry, Centre for Agricultural Research, Hungarian Academy of Sciences, Budapest, Hungary. soil that are physical in nature (e.g., grading and irrigation) tend 4Department of Earth and Planetary Sciences, Johns Hopkins University, to overwhelm native factors (e.g., topography and drainage) and Baltimore, MD. characteristics of soil that take thousands of years to develop. 5Department of Environmental Sciences, University of Helsinki, Lahti, Finland. Moreover, convergence occurs because of differential human 6Unit of Environmental Sciences and Management, North-West University, Potchefstroom, South Africa. effort (e.g., water and nutrient supplements) to overcome environ- 7Department of Ecology, Faculty of Veterinary Science, Szent István University, mental constraints on net primary productivity (NPP) and accord- Budapest, Hungary. ingly soil biological processes—the greater the limitation on these 8Environmental Science and Technology, University of Maryland, College processes, the greater the effort by humans to overcome them. Park, MD. Address for correspondence: Richard V.Pouyat, U.S. Department of Agriculture Hence, in a global comparison of metropolitan areas, the differ- Forest Service, Research & Development, 1400 Independence Ave SW, ence between native and anthropogenic soil will be greatest for cit- Washington, DC 20250. E-mail: [email protected] ies located in biomes with the greatest limitations on NPP or Financial Disclosures/Conflicts of Interest: The idea for GLUSEEN was made decomposition, such as a desert or boreal forest, and for those possible by a Fulbright Specialist grant to R. Pouyat at the University of Helsinki. The GLUSEEN network is supported by a supplemental grant to cities with local soil-forming factors (e.g., parent material) that NSF-ACI 1244820. Soil analyses were supported by the FEKUT grant (SZIE- disproportionately affect soil development (Pouyat et al., 2010; AOTK-KK-UK 12007) of the Hungarian Ministry of Human Resources Pouyat et al., in press). (E.H.). The use of trade, firm, or corporation names in this publication is for The characteristics of urban soils vary widely and are depen- the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department of Agriculture or dent on both direct and indirect effects resulting from urban land- the Forest Service of any product or service to the exclusion of others that use and cover change. Examples of direct effects include soil may be suitable. disturbances such as grading (Pitt and Lantrip, 2000; McGuire, Received April 29, 2015. 2004; Trammell et al., 2011), management inputs such as irriga- Accepted for publication September 9, 2015. Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. tion (Tenenbaum et al., 2006; Zhu et al., 2006), and compaction ISSN: 0038-075X through trampling (Godefroid and Koedam, 2004), whereas indi- DOI: 10.1097/SS.0000000000000125 rect effects include environmental changes such as the urban heat 136 www.soilsci.com Soil Science • Volume 180, Number 4/5, April/May 2015 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. Soil Science • Volume 180, Number 4/5, April/May 2015 Global Comparison of Urban Soils island effect (Savva et al., 2010), atmospheric deposition (Lovett ecosystem convergence hypothesis (Pouyat et al., in press). We et al., 2000; Rao et al., 2014), and changes in plant and animal sampled only surface soils because of the network’s objective to species composition (McKinney, 2006). Early descriptions of ur- relate soil characteristics to soil community structure and decom- ban soils focused mainly on highly disturbed soils exhibiting high position rates that are primarily relegated to surface soil horizons spatial variability, massive structure (i.e., soils having no visible (Swift et al., 1979). Overall goals and objectives of GLUSEEN structure), low organic matter concentration, and contamination are reported by Pouyat et al. (in press). For this study, we address with toxic elements or compounds (Craul and Klein, 1980; the question: How do characteristics of urban soils compare to na- Patterson et al., 1980; Short et al., 1986; Jim, 1993). More recent tive soils at local, regional, and global scales? Specifically, we results show a greater variety of soil conditions that are often compared surface soil characteristics of public greenspace (turf- more favorable for plant growth than the preexisting native soil grass [TUR]), highly disturbed or fill areas (ruderal [RUD]), un- (Hope et al., 2005; Pouyat et al., 2007a; Davies and Hall, 2010; disturbed (remnant [REM]), and native (reference [REF]) soil Edmondson et al., 2012). Adding to the complexity of soil condi- habitat types within and among five metropolitan areas. These soil tions are human desires to maintain cultivated plant communities habitat types roughly corresponded to a continuum of anthropo- that represent the social norms of urban landscapes such as lawns genic effects from relatively low impacts (REF) to those largely and ornamental gardens (Cook et al., 2012; Kendal et al., 2012). impacted by indirect effects (REM), to types that are altered pri- In contrast to increasing heterogeneity, the lag effects of site marily by direct effects, such as in managed (TUR) and drastically history and the propensity of residents to uniformly manage yards disturbed (RUD) areas (Pouyat et al., in press). according to ownership boundaries may actually reduce within We expected surface soil characteristics to “converge” differ- parcel variation