Distinguishing urban soils with physical, chemical, and biological properties
Introduction
Soils are dramatically altered by human activities in urban environments and these alterations distinguish these soils from those in other systems and within urban environments (Craul, 1999). Research has assessed the unique physical, biological, and chemical properties of urban soils. Specifically, urban soil bulk density (Short et al., 1986a, Short et al., 1986b; Jim, 1998a, Jim, 1998b, Jim, 1998c), soil microbial biomass and activity (White and McDonnell, 1988; Carreiro et al., 1999; Zhu and Carreiro, 1999), and soil organic matter quantity and quality (Beyer et al., 1995, Beyer et al., 1996; Pouyat et al., 2002) have been studied and found to be affected by urban conditions.
Physical soil modification is necessary for infrastructure. Road engineering requires packing soils to high bulk densities for load-bearing (Grabosky and Bassuk, 1995). Soil texture amendments, heavy equipment, foot traffic, and covering surfaces with hard-space all reduce aggregation and apply external forces that exceed the soil shear strength resulting in ped breakdown, pore collapse, and dense particle packing (Jim, 1998b). Many studies have measured high bulk densities in urban soils (Short et al., 1986a, Short et al., 1986b; Jim, 1998a, Jim, 1998b, Jim, 1998c). In Hong Kong, Jim (1998a) found two-thirds of sampled soils with bulk density values exceeding 1.6 g cm−3, which is considered the upper threshold for unimpaired root growth (Mullins, 1991). But, substantially lower urban soil bulk densities (e.g., <1.0 g cm−3) have also been measured (Strain and Evans, 1994), which is evidence of the spatial variability of soils in urban environments.
Soil biological properties in urban environments also vary and differ from those in other managed and natural systems (White and McDonnell, 1988; Goldman et al., 1995; Pouyat et al., 1995; Carreiro et al., 1999). White and McDonnell (1988) found significantly lower net nitrogen mineralization rates in the urban forest floor and A horizons (81% and 53%, respectively) than in comparable rural soils. Carreiro et al. (1999) found soil decomposition to be reduced by 25% and microbial biomass by 50% in urban compared to rural soils. Conversely, increased nitrification rates have been found in urban compared to rural soils (Zhu and Carreiro, 1999).
Soil microbial parameters, such as microbial carbon to total organic carbon (Cmic/TOC) or basal respiration to microbial biomass (metabolic quotient, qCO2), are reliable indicators for describing changes in ecosystems (Insam and Domsch, 1988; Insam and Haselwandter, 1989; Insam et al., 1989). The ratio of Cmic/TOC indicates the proportion of carbon that may be readily metabolized. The qCO2 identifies the metabolically active portion of the microbial community (Anderson and Domsch, 1989, Anderson and Domsch, 1990). Disturbance generally produces an initial decrease in TOC and Cmic, and respiration (Kieft et al., 1998). The Cmic/TOC and qCO2 tend to increase immediately following disturbance and then to decrease gradually following recovery. Increases in these ratios are indicative of organic matter losses, decreases indicate organic matter accumulation, and steady-state values are indicative of climax communities (Insam and Haselwandter, 1989). Beyer et al. (1995) found higher Cmic/TOC and the qCO2 values for younger urban soils indicative of early soil development. Although important for examining soil biological and organic matter dynamics, these microbial parameters have been rarely studied in urban soils (Beyer et al., 1995).
The quantity and quality of urban soil organic matter is quite variable (Beyer et al., 1996; Carreiro et al., 1999; Pouyat et al., 2002). In preparation for infrastructure topsoil is removed, leading to reductions in soil organic matter (Pulford, 1991; Craul, 1993; Harris et al., 1999). The removal of grass clippings, tree leaves, and other organic debris can further reduce inputs to the soil organic matter pool; while, organic additions such as top soil replacement, mulch, root turnover, microbes, earthworms, grass clippings, and leaf litter left on site help to build soil organic matter (Craul, 1999). Studies of urban soils often describe decreased organic matter contents compared to soils of other systems (Cotrufo et al., 1995; Zhu and Carreiro, 1999). Organic matter contents in urban soils were less than 1%, compared to forested soils with 4–5%, and some agricultural soils with as much as 10% soil organic matter (Craul, 1993; Jim, 1998a). In contrast, organic matter quantities have been measured in urban soils that are substantially higher than in other non-urban soils (White and McDonnell, 1988; Pouyat et al., 1995, Pouyat et al., 2002). Pouyat et al. (2002) described urban soils with significantly higher organic carbon densities, 97 g kg−1, than suburban, 83 g kg−1 and rural soils, 73 g kg−1.
Most research of urban soil organic matter has been quantitative, but qualitative differences have also been measured (Beyer et al., 1995, Beyer et al., 1996). Certain portions of soil organic matter are more labile than other more recalcitrant fractions. Particulate organic matter (POM) has been identified as soil organic matter more actively involved in decomposition and mineralization processes (Elliot and Cambardella, 1991; Magid et al., 1996; Six et al., 2002). Although POM is known to be indicative of soil organic matter quality, no research has examined POM in urban soils.
The goal of our study was to emphasize and explain spatial variability in urban soils. We attempt this by measuring physical, chemical, and biological properties of soils in an array of urban landscapes. We then relate these properties to soil processes and forming factors to solicit differences in the soil development in various urban landscapes. The research database on soil development in urban soils is minimal, and we hope to contribute knowledge to this growing area of interest.
Section snippets
Materials and methods
Study sites were selected within the cities of Moscow, Idaho, 46°44′N and 116°58′W and Pullman, Washington, 46°43′N and 117°11′W. Human population density in Moscow, ID is 882 people km−2 and 1069 people km−2 in Pullman, WA. The soils are described as Palouse silt loam (fine-silty, mixed, mesic, Pachic Ultic Haploxerolls) with 7–25% slopes (USDA Soil Survey of Whitman County, Washington, 1980; USDA Soil Survey of Latah County, Idaho, 1981). The soils were formed in loess and volcanic ash and are
Results
All soils were classified as silt loam (Table 2). The urban landscapes did not differ in silt, but clay contents were significantly greater on new residential (21.0%) and mulched (23.3% and 19.3%) sites compared to old residential (12.0%), street (13.0%), and park (11.0%) sites. New residential (17.3%) sites had significantly lower sand contents compared to old residential (27.0%), park (26.5%), and street (30.5%) sites. Sand contents on mulched (18.5% and 18.3%) sites tended to be closer to
Discussion
Soil development occurs through interactions of climate, organisms, relief, parent material, and time (Jenny, 1941). Of the developmental factors, time played the most significant role in physical, chemical, and biological differences among these urban soils. Old residential, street, and park sites had substantially longer times since initial disturbances (mean of 64 years old). Conversely, new residential, old mulch, and new mulch landscapes were all less than 20 years old (mean of 9 years
Conclusion
Urban soils are distinguishable within urban environments. Physical, chemical, and biological properties are modified by a significant site disturbance for urban infrastructure. Time is an important soil developmental factor for distinguishing soils in urban environments. Urban landscapes with increased time since initial disturbance have been found with reduced soil bulk densities, increased biological activity, and increased soil organic matter. Future research should examine urban soil
Acknowledgments
The authors thank Richard Gill, Ph.D., Donald Thill, Ph.D., Robert Tripepi, Ph.D., and the anonymous reviewers for their suggestions. We also thank the ISA Research Trust (John Duling Grant) and the University of Idaho Seed Grant for their support of this project.
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