Explaining soil organic carbon sequestration in an urban ecosystem

Background/Question/Methods

The reality of climate change warrants investigation into carbon sequestering processes. Research into the mechanisms of soil carbon storage is valuable because most of the carbon on Earth is stored in soils. Explication of soil carbon dynamics in urban landscapes is particularly pressing as the sizes and number of cities rapidly rise. Urban areas experience unnaturally high temperatures due to the urban heat island effect and high soil water content and levels of nitrogen as a result of management. The expected effect of such abiotic conditions is decreased soil carbon due to increased respiration, but several recent studies have found enhanced carbon sequestration in urban environments. The aim of this study is to examine the factors influencing soil carbon storage in an urban ecosystem by quantifying soil carbon and the abiotic, biotic and anthropogenic variables in Portland, Oregon. The A-horizon of four ecodistricts was sampled; soil organic carbon, total soil carbon, total nitrogen and C:N ratio were quantified and correlated to a suite of urban variables.

Results/Conclusions

Here we present the relationships among abiotic (water content, nitrogen, depth, temperature), biotic (vegetative cover, microbial and root biomass) and anthropogenic (population density, percent impervious surface coverage) variables and soil organic carbon concentration. Of particular interest are those factors under the control of citizens.  Homeowners’ decisions regarding water consumption, fertilization practices and type, amount and distribution of vegetative cover directly influence carbon deposition into and cycling within the soil. We found that factors associated with vegetative cover were major influences on soil organic carbon accumulation in this system. Results from our research indicate that the environmental factors specific to urban areas are crucial variables to consider when modeling soil carbon dynamics within cities. The results of this study are applicable to ecodistrict boundary formation, soil carbon cycling and carbon budgeting in urban landscapes.