Urbanization has led to an increase of combustion-derived, organic carbon emissions that can deposit diffusely to soils within and surrounding cities. One of the most common types of urban pollutants, Polycyclic Aromatic Hydrocarbons (PAHs), may be toxic and are detrimental to environmental quality. Despite this concern, there is a considerable gap in our knowledge about the environmental fate of PAHs in highly populated urban areas. In contaminated point source sites, certain microorganisms are known to utilize petroleum-based pollutants, yet little is known about the specific metabolic capacities of urban soil microbes that can degrade diffuse carbon compounds deposited from the atmosphere. Furthermore, microbial community structure and function in a desert low in organic content may be altered by exposure to anthropogenic organic compounds. In our research, we ask, what is the magnitude, distribution, and fate of non-point carbon pollution in an arid urban ecosystem? To answer this question, we collected 83 soil samples near highways across the Phoenix valley to characterize and quantify PAH compounds using gas chromatography-mass spectrometry (GC-MS). Additionally, we measured a suite of soil properties and processes to explore the role of abiotic and biotic factors on the retention of PAHs in urban soil.

PAH concentrations in arid Phoenix soils ranged from 52 ug/kg dry soil to 8,296 ug/kg (mean 926 µg/kg), nearly an order of magnitude lower on average than expected based on data from other cities. Although the most likely source for PAH content in roadway soils is vehicle emissions, they were not correlated with traffic density (r2 = 0.024; p = 0.26) or highway age (r2 = 0.044; p = 0.13) across all sites. However, PAH concentrations were significantly correlated to soil organic matter content (r2 = 0.36; p < 0.001). These results suggest that PAH concentrations in roadway soils of desert cities may be controlled by factors associated with carbon retention, such as soil organic matter, rather than source or rate of deposition.