- Rich Pouyat, United States Forest Service
- Ian Yesilonis, University of Maryland
- Dave Nowak, United States Forest Service
- Peter Groffman, Institute of Ecosystem Studies
On a global scale, soil C pools are roughly three times larger than the C stored in all land plants. At this scale, soil C pools are primarily a function of the inputs of organic matter to the ecosystem (net primary productivity or NPP) and the average rate of decay within the ecosystem (soil heterotrophic respiration), both of which are controlled by environmental factors such as soil temperature and moisture. Due to differences in sensitivity between decay rate and NPP, there is a wide variation in organic soil C pools on a global scale. There is great current interest in the question of whether soil C pools will increase or decrease when global warming occurs and whether various land use changes and their associated soil modifications will affect soil C storage.
Very little data are available to assess whether urbanization leads to an increase or decrease in soil C pools. This paucity of data has made it problematic to predict or assess the regional effects of land use change on soil C pools in various regions of the world. As land is converted to urban uses there are direct and indirect factors that can affect soil C pools. Direct effects include physical disturbances, burial or coverage of soil by fill material and impervious surfaces, and soil management inputs (e.g., fertilization and irrigation). Indirect effects include changes in the abiotic and biotic environment as areas are urbanized. The direct effects often lead to new soil parent material on which soil development then proceeds. Indirect effects that can influence this development as well as processes in intact soils include, the urban heat island effect, soil hydrophobicity, introductions of exotic plant and animal species, and atmospheric deposition of various pollutants. Moreover, toxic, sub-lethal, or stress effects of the urban environment on soil decomposers and primary producers can significantly affect soil C fluxes.
Current BES research is addressing how soil organic carbon pools vary across different land-use types in urban landscapes. Plots were located in Baltimore City by a stratified random design. Seven land-use types were delineated using remotely sensed data and were weighted based on the aerial coverage of each type. These land-use types included commercial; industrial; institutional; transportation right-of-ways; high, medium, and low density residential; and forest. A grid was laid over the land-use map and 200 plots were randomly located on the grid until the plot locations were distributed throughout the land-use and cover types.
At each selected location a circular plot with an 11.35 m radius was established. In a previous study, plant species, vegetation structure, and other measurements were recorded in each plot (D. Nowak, unpublished data). During the summer of 2000, 127 of the original 200 plots were sampled for soils. This occurred because many of the plots landed on impervious surfaces, or permission was not granted to collect soil samples at a number of private residences. If more than one type of cover occurred within a plot, samples were stratified by the cover types present. Cover types included tree, managed grass, unmanaged herbaceous, and "no vegetation" categories. Two techniques were used to acquire soil samples within the plots: an undisturbed 5 cm bulk density core (3 per cover type) and a composite soil sample to a depth of 15 cm with a 2-cm diameter stainless steel sampling probe. Typically, 10 to 15 cores (2-cm) were composited per plot in a grid pattern, depending on the amount of soil surface that was exposed. The composite sample was air-dried and sieved in the lab with a 2 mm mesh sieve. The undisturbed cores were used to determine soil bulk density and moisture retention (the latter measurement for another study). The dry sieved samples were used to measure various soil properties, of which organic matter content will be reported here. Organic matter concentration of mineral soil was determined by loss on ignition (450° C for 4 hr.). Soil organic C was estimated by multiplying organic matter content by the Van Bemmelen factor (0.58). Organic C amounts (kg C m-2) in the mineral soil were calculated the same as for the urban-rural gradient study described above except the depth used was 15 cm and the fraction of coarse fragments (> 2 mm) were not factored into these calculations. Estimates of large rocks, boulders, and other large objects were not made for these plots.
Organic C amounts in exposed (i.e., not covered by impervious layers) surface soil (0-15 cm) varied widely across land-use types in Baltimore City (Fig. 1). Both C density and concentration responded to land-use in the same way, which may be due to the absence of coarse fragment data, which may vary considerably across soils in these land-use categories (Fig. 1 and Table 1). Soil organic C was highest in low density residential and institutional land-use types; however, these areas exhibited the greatest variation in C density (Fig. 1). Low density residential and institutional land-use types had 44 and 38% higher organic C densities than the commercial land-use type, respectively. Forest cover, medium density and high density residential, and transportation rights-of-way had intermediate organic C densities (Fig. 1).
