Effect of the tropical grass Brachiaria brizantha (Hochst. ex A. Rich.) Stapf on microbial population and activity in petroleum-contaminated soil

The effect of the tropical pasture grass Brachiaria brizantha on numbers of bacteria, fungi and degraders of alkanes, aromatics, cycloalkanes and crude oil in petroleum hydrocarbon contaminated and uncontaminated savannah soil was evaluated. Substrate induced soil respiration and soil pH were compared between planted and unplanted soil. B. brizantha had a mostly increasing effect on microbial numbers. As an exception, growth of bacteria was not or negatively affected. Microbial respiration and pH were always lower in planted than in unplanted soil. Low pH may result from enhanced oil degradation in planted soil leading to an accumulation of organic acids. A comparable stimulation of crude oil degraders and fungi in planted soil points to the importance of fungi. Since they tolerate lower pH values than bacteria, they are considered to play a central role in oil degradation. Given that the enhancement of crude oil degradation under the influence of B. brizantha could not clearly be correlated to microbial numbers and activity, other factors like oxygen availability, plant enzymes and synergistic degradation by microbial consortia have to be considered.