Realizing the opportunities of black carbon in urban soils: Implications for water quality management with green infrastructure

Highlights: • We assessed surface and subsurface soil BC concentrations in 11 US cities. • Soil BC is influenced by surface horizon thickness, traffic, and vegetation density. • Native urban soil can regulate contaminant cycling in passive green infrastructure. Soils and associated microbial processes regulate the carbon cycle and provide a sink for atmospheric black carbon (BC). Particularly in urban areas, present and accumulated soil BC may act as an effective sorbent of anthropogenic contaminants in green spaces. We characterized carbon concentrations that have accumulated in urban soils (organic carbon, BC, and inorganic C) and determined soil physical attributes (soil texture, hydraulic conductivity) from urban soil assessments (surface and sub-surface horizons) carried out in eleven cities in the United States. We used both ordinary least squares and non-parametric classification and regression tree (CART) methods to discern trends in soil BC concentrations with regard to soil, landscape, and emission characteristics. We found that for all cities, regional traffic density and vegetation were good predictors of soil BC concentration. Additionally, the thickness of the top soil horizon explained additional variation in sub-surface BC concentrations. Sites with coincident BC stocks and favorable infiltration rate were discussed as per their potential for improving water quality in multifunctional green infrastructure installations. In the broader sense, the high sorption capacity of existing, accumulated soil BC can contribute to regulation of contaminant cycling in urban areas and may enhance the overall value of urban soils in terms of ecosystem services.