Carbon transformations by indigenous microbes in four hydrocarbon-contaminated soils under static remediation conditions

We sought to learn about the transformations of hydrocarbons and limitations to bioremediation in four hydrocarbon-contaminated soils. Two soils were contaminated with creosote and two with petroleum. We incubated them either with or without added N, P, K and S. We monitored CO2 evolution, and residual dichloromethane-extractable organic C (DEO-C) after 10 wk. Indigenous populations were active in all soils. A single-component first-order model fit the CO2 respiration rate data, yielding estimates of potentially mineralizable C (C-o), and specific decay rate, k. The ratio C: DEO was lower in heavier textured and strongly aggregated soils compared with the more poorly aggregated sandy soils. Low respiration rates in the more clayey soils were related to low C-o rather than to k for the available C. In the highly amended soils the loss of total C approximated the production of CO2-C while the loss of DEO-C was greater than the evolution of CO2-C. We conclude: 1) Under circumstances such as hydrocarbon contaminants with long exposure to the soil, static systems may be sufficient for metabolism of available contaminants by indigenous microorganisms. 2) Increases in clay content and stability of aggregates, together with biotreatment to remove hydrocarbons may reduce bioavailability of residual contamination. 3) In soils with high clay content, contaminant transformations or attenuation without production of CO2 may be substantial.