Ian D. Yesilonis

A global comparison of surface soil characteristics across five cities: a test of the urban ecosystem convergence hypothesis

Abstract As part of the Global Urban Soil Ecology and Education Network and to test the urban ecosystem convergence hypothesis, we report on soil pH, organic carbon (OC), total nitrogen (TN), phosphorus (P), and potassium (K) measured in four soil habitat types (turfgrass, ruderal, remnant, and reference) in five metropolitan areas (Baltimore, Budapest, Helsinki, Lahti, Potchefstroom) across four biomes. We expected the urban soil characteristics to “converge” in comparison to the reference soils. Moreover, we expected cities in biomes with more limiting climatic conditions, or where local factors strongly affect soil characteristics, would exhibit the greatest variance across soil types within and among cities. In addition, soil characteristics related to biogenic factors (OC, TN) would vary the most because of differences in climate and human efforts to overcome limiting environmental conditions. The comparison of soils among and within the five cities suggests that anthropogenic, and to a lesser degree native, factors interact in the development of soils in urban landscapes. In particular, characteristics affected by anthropogenic processes and closely associated with biogenic processes (OC, TN) converged, while characteristics closely associated with parent material (K, P) did not converge, but rather diverged, across all soil habitat types. These results partially supported the urban ecosystem convergence hypothesis in that a convergence occurred for soil characteristics affected by climatic conditions. However, the divergence of K and P was unexpected and warrants adjusting the hypothesis to account for variations in anthropogenic effects (e.g., management) that may occur within soil habitat types impacted by humans.

Carbon Stocks in Urban Forest Remnants: Atlanta and Baltimore as Case Studies

Urban environments influence carbon (C) and nitrogen (N) cycles of forest ecosystems by altering plant biomass, litter mass and chemistry, passive and active pools of C and N, and the occurrence and activity of decomposer organisms. It is difficult to determine the net effect of C storage due to the number of environmental factors exerting stress on urban forests. Using a conceptual model to synthesize results from gradient studies of forest patches in metropolitan areas, we attempt to explain the mechanisms affecting C cycling. We also assess the relative importance of C accumulation in urban remnant forests with respect to other land uses previously disturbed or managed. The cities of Baltimore and Atlanta are used as case studies. The C density of forest above-ground biomass for Baltimore City, 8 kg m-3, and Atlanta, 10. 6 kg m-3, is significantly higher for both medium- and high-density residential areas. Baltimore City has a forest-soil C density of 10.6 kg m-3, a below-to-above ground ratio of 1.3. Urban forest remnants in these two cities store a high amount of C on a per-unit basis both above- and below ground relative to other land uses, but total C storage is lower due to the lower acreage of urban forest in these cities relative to other land uses.