Black carbon (BC) is produced from the incomplete combustion of fossil fuels and biomass. It has been detected in many important geochemical pools (soils, sediments, aerosols) and has been shown to be a significant portion of soil organic matter in some locations (45% in frequently burned soils). BC is ubiquitous in nature and its presence in long–term carbon reservoirs is well documented. Uncertainties exist in the BC global and regional budgets with respect to sources and sinks. The terrestrial soil sink is poorly constrained due to the lack of data. This study focuses on BC in soils from the greater-Phoenix area in central Arizona. Sixty-three soil samples ranging from rural desert soils to urban soils were analyzed for both BC concentration and BC isotopic composition. Sizable inputs of BC occur from both fossil fuel combustion and biomass burning in this region, making it a good location to investigate black carbon.
Black carbon concentrations in soil were determined by chemo-thermo oxidation and detected using an isotope-ratio mass spectrometer. Results show that BC comprises between 0.02 and 0.78 % of the total soil mass and 1.65 to 62.86 % of the soil organic carbon (by weight). These results indicate that BC in central Arizona is a significant portion of the soil organic carbon (SOC) pool. In addition, urban locations have a higher concentration of BC than urban-fringe and rural soils. Since a major source of BC is the burning of fossil fuels (typically from automobiles), it is not surprising that BC concentrations are higher in urban areas because of the increased presence of automobiles. The ¹³C isotopic composition of most black carbon soil samples fall within the range from -10 to -20 ‰. The bulk organic matter in the region has an isotopic composition between -20 and -24 ‰. This is in contrast to published results from Lohmann et al. (2009) suggesting that black carbon and the bulk organic carbon counterpart have similar isotopic signatures. The cause of this isotopic fractionation is as yet unknown and could be due to microbial degradation or to fractionation during combustion.