Quantification of black carbon in urban soils

Black carbon (BC), which comprises partially combusted organic material such as char and soot, is a hazardous pollutant and contributor to global warming, but is also highly stable, and once deposited in soil contributes to long-term stocks of sequestered carbon. The effects of land use and proximity to emission sources on the quantities of BC stored in soil have heretofore received little scientific attention. Quantification of BC in soils has proven problematic as it is difficult to chemically or physically distinguish between thermally altered (BC) and unaltered organic carbon fractions. This thesis presents studies leading to the development of an improved method for soil BC measurement, employing chemooxidation, which was rigorously tested using additions of a range of major types of BC to soil. The method was applied to measure soil BC in gardens and non-domestic greenspaces, under grass and tree cover in a representative UK city (Leicester), and in agricultural land within 7.5 km of the city. Soil BC was also measured in transects away from one of the main roads in the city, to determine whether vehicle emissions interacting with tree cover, potentially intercepting particles, can locally affect BC concentrations. BC provided 28.1 % of the total soil carbon in the city, twice the concentration found in agricultural land. Soil BC was negatively correlated to distance from the main road, implicating vehicle emissions as important local sources, but effects of trees on this relationship were inconclusive. Positive correlations were found between soil BC and non-pyrogenic soil organic carbon concentrations, soil nitrogen, and the soil carbon to nitrogen ratio, suggesting that BC may increase recalcitrance and sequestration of organic carbon. It is concluded that BC represents a significant and distinct carbon pool in urban soils, and may have a considerable influence on soil biogeochemical cycling and carbon sequestration.