Phytoremediation of oil contaminated soil by some arid legume tree species

Petroleum contamination of soil is a serious problem throughout the world. Phytoremediation is a promising technology for the clean up of petroleum contaminated soil. In the present work the rhizosphere of Prosopis cineraria (L.) Druce, Acacia senegal (L.) Willd. and Acacia nilotica (L.) Willd. Ex. Del. plants were tested for their abilities to stimulate the microbial degradation of soil pollutants in desert soil contaminated with 2.5-2.6% crude petroleum oil. The results showed that the roots of the three different plants were density associated with total bacteria, fungi and oil-degrading microorganisms, this is confirmed from the (R+/S+) ratios which ranged from 55.2-250.8 (for total bacteria), 20-131.3 (for fungi) and 95.7-296.1 (for oil degraders). Percentages of oil-degraders were higher in the rhizosphere soil of P. cineraria (65.5%) as compared to the rhizosphere soil of A. senegal and A. nilotica plants (22.5 % and 20.2 % respectively). The results of the biodegradation of oil and its fractions showed that great reduction (26 %) of total petroleum hydrocarbons (TPHs) was observed in the rhizosphere soil of P. cineraria as compared to 15.6 % and 12.8 % reduction in rhizosphere soil of A. senegal and A. nilotica respectively. It was observed also that in the polluted non-cultivated soil the TPHs were reduced by 8.2 - 10.5 % as a result of biostimulation process only (addition of nutrients). The results also showed that P. cineraria rhizosphere was able to reduce more of the saturated (43.0 %) and more of the aromatics (25.7 %) fractions, compared to (35.2 % and 7.9 %) for A. senegal and (31.2 % and 4.1 %) for A. nilotica rhizospheres. It is of interest to find that 5.3 % of the hardly degradable fraction resins were degraded in rhizosphere soil of P. cineraria. The present results clearly demonstrated that P. cineraria provided successful phytoremediation process of a contaminated desert soil as compared to the other two legume trees.