The storage and dynamics of urban soil carbon

Background/Question/Methods

The amount of carbon (C) stored in soil over time, or C sequestered, is a balance between C input through net primary productivity (NPP) and loss through decay (soil heterotrophic respiration), both of which are controlled by environmental factors, including soil  temperature and moisture, nitrogen availability, and litter chemistry. Carbon sequestration in urban soils is an important process that helps to mitigate the effects of increased emissions of greenhouse gases into the atmosphere that occurs in urban areas. However, the gain or loss of C from soil can be greatly affected by urban land use and urban environmental change. Urban land-use change can affect biogeochemical cycles through altered disturbance regimes, landscape management practices (e.g., irrigation and fertilization), built structures and impervious areas, and changes in environmental factors (heat island effect, pollution, introduction of non-native species, and loss of native species). Many of these effects are dependent on human decision making, e.g., whether to maintain lawns or not, and have in some cases the potential to sequester more C than soils in the previous native ecosystem. 

Results/Conclusions

The data collected thus far suggests global urbanization can lead to both an increase and decrease in soil C pools and fluxes, depending on the native system impacted by urban development.  Moreover, results suggest that soil C storage in urban ecosystems is highly variable with very high and low C densities (kg m^-2 to a 1 m depth) present in the landscape at any one time. For biomes in which soil C stocks are relatively low, e.g., warm dry climates, urban land-use management results in higher soil C densities; whereas, in cooler more moist climates urban land-use change results in an overall loss of soil C. For those soils with low SOC densities, there is potential to increase C sequestration through management, but specific urban related management techniques need to be evaluated for their effectiveness in storing C and for reducing emissions of greenhouses gases.  In lawn areas that are fertilized and clippings are returned, the maximum amount of C stored in lawns appears to asymptote at around 35-45 years. Using CENTURY modeling runs, it appears that having trees in the landscape, in combination with intermediate management levels (clipping, fertilization), results in the highest level of soil C accumulation. Finally, recent observations of soil C pools beneath impervious surfaces have found relatively low C densities; however, densities that are twice as much as originally thought.