Impact of crude oil on functional groups of culturable bacteria and colonization of symbiotic microorganisms in the Clitoria-Brachiaria rhizosphere grown in mesocosms

Abstract

This research evaluated the changes on populations of culturable N-fixing free bacteria (NFFB) and P-solubilizing bacteria (PSB), as well as on the root nodulation by native rhizobia, the root colonization and spore number of arbuscular mycorrhizal fungi (AMF), in the rhizosphere of Clitoria ternatea and Brachiaria brizantha grown in mesocosms contaminated with crude oil (0, 3000, 6000, 9000, and 12000 mg kg-1), for 240 days. After 24 h of soil contamination, the highest populations of NFFB and PSB (5.5 and 4.9LogUFC, respectively) were found in control, and the lowest populations were obtained at 12000 mg kg-1 (5.1 and 4.2LogUFC, respectively). In contrast, at 60 and 240 days, the control showed lower populations of NFFB and PSB (5.4 and 4.8LogUFC, respectively) than contaminated treatments. The highest number of root nodules in C. ternatea was quantified in control at 60 and 240 days (25 and 27 nodules, respectively) in comparison to those observed at the treatment with 12000 mg kg-1 (7 and 1 nodule, respectively). At 60 days, AMF colonization in both plant species, and the number of spores significantly decreased as the crude oil concentration increased; however, at 240 days, the highest number of AMF spores was recorded at treatments with 6000 and 12000 mg kg-1. The dry weight of both plant species significantly decreased as crude oil concentrations increased. Although C. ternatea was more susceptible to the toxic effects of crude oil, this plant species showed greater content of total chlorophyll than B. brizantha.