Microbiological resilience of soils contaminated with crude oil

The effects of simulated crude oil soil contamination on microbial biomass, its activity and capacity to degrade crude oil hydrocarbons were examined in different soil types under different crop managements. The effect of the addition of different organic amendments (glucose, maize stalks and maize stalk compost) on microbial survival and crude oil degradation was also investigated. Microbial biomass was measured by the fumigation-extraction method and its activity was monitored by CO2 evolution. Soil resilience was evaluated also from the amount of crude oil degraded at the end of the incubation (60 days at 25 degreesC). This ranged between 1% and 45% in unamended soils with an average of 31.3 +/- 3.8% and increased up to 36.4 +/- 2.1% in compost amended soils, although this difference was not significant. Less than 2% of crude oil was mineralized to CO2 during 60 days of incubation, whereas more was irreversibly adsorbed (36-50%) and a smaller portion was lost by volatilization (10-33%). Crude oil caused a marked decrease in microbial biomass in Inceptisols (-25%), a small increase in Entisols (+5%) and a much larger increase in Mollisols (+53%). In Mollisols, microorganisms were better protected from the adverse effects of oil spills, and oil hydrocarbons were utilized as a carbon source, whereas in Inceptisols and Entisols, crude oil adversely affected the soil microbial populations, suggesting less resilience. Grassland soils, generally, better sustained the microorganisms in comparison to similar arable soils under maize monoculture. The addition of organic substrates (glucose, maize stalks and maize stalk compost) to contaminated soils bad no synergistic effect on the decomposition of crude oil components but produced a marked increase in microbial biomass (21-36%). This increase was smaller than in uncontaminated soils (range 27-75%). The metabolic quotient increased markedly in contaminated soils from a mean of 1.24 up to 4.78 mg CO2-C g(-1) Bc h(-1). Compost was the most effective amendment in reducing the metabolic quotient (2.36 mg CO2-C g(-1) Bc h(-1)), indicating a decrease in stress conditions caused by oil contamination. In Mollisols, the qCO(2) increased much less than in other soil types. No effects were observed on the decomposition rate of the different amendments indicating that decomposition of added substrates in soil was not significantly modified by crude oil contamination. (C) 2003 Elsevier B.V. All rights reserved.