A framework for soil carbon management in cities: Linking urban land management to carbon stocks and fluxes

Background/Question/Methods

Urban areas, source of more than half of global CO₂ emissions, are projected to triple between 2000 and 2030. A significant proportion of urban carbon (C) fluxes are mediated by biogenic pathways, which are shaped by the interaction of human decision-making and the ecoregion. This interaction is key to understanding land use change effects on global biogenic C stocks and fluxes. The recent 10-fold increase in urban soil C literature combined with existing aboveground C data represents a novel opportunity to analyze these relationships for U.S. metropolitan areas. We present a framework for urban soil C management based on a meta-analysis of U.S. urban soil C data and a case study at the site-management scale. Our framework allows for the analysis of the following hypotheses: 1) the net change in above- and below-ground urban C stocks after land use change varies by ecoregion; 2) the net change links to urban metrics including land use history, management, plant biodiversity, and socioeconomic factors; and 3) human decisions and activities (e.g., management) will influence anthropogenic and biogenic C fluxes at multiple scales. The findings from this synthesis study will provide guidance on urban planning and soil C management for policy- and decision-makers.

Results/Conclusions

The case study demonstrated that rehabilitation (e.g., through subsoiling and compost incorporation) of a soil degraded by land development  resulted in increased soil C storage in lower soil regions after five years and offset approximately 50% of the C loss that occurred during land development. Yet, greenhouse gas emissions increased by 150%. These results show management can affect soil C stocks and fluxes significantly. With the framework we developed, these stocks and fluxes can be scaled up to city, region, and global levels. Our framework identifies the initial soil C stocks, ecoregion, urban metrics, human management (e.g., development pattern, mulching, and irrigation) and current C stocks and fluxes. The initial C data of pre-developed lands is based on published data bases from multiple agencies and consortiums. Differences between ecoregions after urban development will combine pre-development soil C levels with post-development C stocks determined from a literature review. For example, we found soil C change ranges from -7.3 to 3.5 kg m^-2 in five U.S. cities (Atlanta, Baltimore, Boston, Chicago, Oakland, and Syracuse) across different ecoregions. This framework allows for scenario simulation and sensitivity analyses to test the effects of human decisions and activities on soil C stocks and fluxes.