Mapping soil lead speciation and soil microbial response at an abandoned firing range in Oak Ridge, TN

Background/Question/Methods

Lead (Pb) contaminated soils represent severe environmental problems as well as a significant hazard to human health.  Prolonged exposure to high levels of toxic metals exerts considerable selective pressure on the soil microbial community and alters soil and ecosystem health.  The objective of this study is to understand how the total level of Pb contamination, as well as Pb speciation in these soils, has affected the soil microbial community structure in terms of fungal:bacterial ratios at an abandoned small-arms firing range in east Tennessee.

Ninety-two soil samples were collected in a spatially explicit sampling design across two basic ecosystem types, grassland and forested, surrounding the backstop at the abandoned firing range.  Soils were analyzed via qPCR for fungal:bacterial ratios.  Total Pb was determined by XRF and sequential extractions were performed to determine lead fractionation and binding in these soils.  

Results/Conclusions

Soil Pb concentrations ranged from 23.6 mg kg^-1 to over 2500 mg kg^-1 with the highest total lead concentrations geographically located nearest the firing backstop and the lowest lead in the soils furthest from the backstop.  Soil fungal:bacterial ratios were more tightly correlated to soil lead speciation than to total soil lead, however.

Many species of fungi are capable of transforming insoluble metal compounds into soluble derivatives, an environmentally important process when these fungi constitute a significant portion of the overall soil community.  Understanding how soil microbial community structure responds to soil lead content and the dynamic biogeochemical impacts the soil fungal community may exert on extant lead in soils will yield a greater understanding of lead bioavailability in soils and human risk assessment.