Abstract

Urban areas generate about 77%, worldwide, of the carbon (C) based compounds that are annually released to the atmosphere, whether in the form of gases (e.g., 71% of CO2, 60% of CO) or fine aromatic C solids (70%), all of which may pose environmental risks, from biological cell to global levels (e.g., air quality, scattering light). Those C-rich atmospheric particles, referred to as “particulate matter and black carbon” (PM-BC), as opposed to aquatic origin “particulate organic carbon” (POC), are present as finer than 2.5 µm and produced during incomplete combustion processes such as in vehicle emissions, domestic heating and biomass burning. However, while the post combustion C gases remain for a while in the aerial environment, PM-BC undergoes a relatively brief cycle of emission-dispersion-transformation-deposition. Globally, these account for about 47 Tg C y⁻¹ from wet deposition of PM-BC, and 400 Tg C y⁻¹ from deposition of dissolved organic carbon (DOC) in rainfall, which generally disturbs the global C cycle, and specifically those of urban centers and surrounding areas, permanently shifting the C budget of the soil, and altering its chemistry, which is important because the soil is considered, by far, as the main C sink in terrestrial ecosystems. However, once the deposited C begins interacting with soil components, its negative effects are not necessarily measurable in the soil environment. On the contrary, soils in which PM-BC is deposited may produce a negative bioenergy that subsequently shifts the biogeochemical C reactions towards those materials that are composed of more stable organic molecules, thereby increasing the recalcitrant C pool, and likely contributing to other soil organic C (SOC) stabilization mechanisms that increase C sequestration in soil. In addition, PM-BC plays an important role as a strong sorbent for common urban pollutants (e.g., trace elements and persistent organic pollutants). Therefore, the focus of this review is to synthesize the previously published reports of the effects of PM-BC on the physico-chemical mechanisms of long term C storage in soils.