.|  Baltimore Ecosystem Study
Soil Research Projects

Carbon Densities in Urban Soils
  • Rich Pouyat, United States Forest Service
  • Ian Yesilonis, University of Maryland
  • Dave Nowak, United States Forest Service
  • Peter Groffman, Cary 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.
 
BES research is addressing how soil organic carbon pools vary across different land-use types in urban landscapes specifically residential lawns and forest at the national and city scale. Residential lawns from Baltimore and Denver were studied to determine if turf grass systems would be similar in soil organic C densities in urban areas despite differences in climate, parent material, and topography (Pouyat et al., 2009). It was found that Baltimore and Denver had similar soil organic carbon densities (Fig 1), possibly due to greater management efforts in the drier Denver region. This supports the idea that anthropogenic factors such as management supplements of nutrients and water overwhelm native environmental factors that control soil organic C storage.
 

Figure. 1 Mean (+/-SE) SOC densities (1-m and 0- to 20-cm depths) and the proportion of SOC in the 0- to 20-cm depth of residential soils in Baltimore (n=26) and Denver (n=13) metropolitan areas. Residential SOC density measurements calculated from undisturbed cores. P values represent comparisons of SOC medians using a Kruskal-Wallis test

In addition to lawns, urban forests were also studied. For the above-ground biomass, Baltimore's urban forest was projected to decline in both number of trees and canopy area over the next century (Nowak et al., 2004). However, these urban forest of store a high amount of C on a per-unit basis both above- and below- ground relative to other land uses (Fig 2) (Yesilonis and Pouyat, 2012). The total C storage is lower due to acreage of urban forest relative to other land uses.
 

Figure. 1 Mean (+/-SE) SOC densities (1-m and 0- to 20-cm depths) and the proportion of SOC in the 0- to 20-cm depth of residential soils in Baltimore (n=26) and Denver (n=13) metropolitan areas. Residential SOC density measurements calculated from undisturbed cores. P values represent comparisons of SOC medians using a Kruskal-Wallis test

At the national scale, based on 10 USA cities including Baltimore, urban trees store 700 million tons of carbon, with an annual sequestration rate of 22.8 million tons C/year (Nowak and Crane, 2002). Below-ground storage was found to store more carbon than the trees (Pouyat et al., 2006). The ratio of above-ground to below-ground estimates of C storage was 2.8. Residential lawns store a great amount of carbon because of the management inputs.
 
Carbon Densities in Urban Soils
References:
 
Nowak, D.J., Crane, D.E., 2002. Carbon storage and sequestration by urban trees in the USA. Environmental Pollution 116, 381-389.
 
Nowak, D.J., Kuroda, M., Crane, D.E., 2004. Tree mortality rates and tree population projections in Baltimore, Maryland, USA. Urban Forestry Urban Greening 2, 139-147.
 
Pouyat, R.V., Yesilonis, I.D., Golubiewski, N.E., 2009. A comparison of soil organic carbon stocks between residential turf grass and native soil. Urban Ecosystems 12, 45-62.
 
Pouyat, R.V., Yesilonis, I.D., Nowak, D.J., 2006. Carbon Storage by Urban Soils in the United States. Journal of Environmental Quality 35, 1566-1575.
 
Yesilonis, I., Pouyat, R., 2012. Carbon stocks in urban forest remnants: Atlanta and Baltimore as case studies, in: Lal, R., Augustin, B. (Eds.), Carbon Sequestration in Urban Ecosystems. Springer, pp. 103-120.