**4. Conclusion**

**3.3. Effect on soil microorganisms**

96 Environmental Risk Assessment of Soil Contamination

affect soil microbiota [11, 4].

0.0E+00 5.0E+05 1.0E+06 1.5E+06 2.0E+06 2.5E+06 3.0E+06 3.5E+06 4.0E+06 4.5E+06

rhizospheric soil.

**Microorganism (CFU g-1)**

the soil as saprophytes, until they infect a radical hair.

*Crotalaria incana*

150 79,457

**Concentration (mg.kg-1)**

In plants *Leucaena leucocephala* high concentration of weathered oil did not affect populations of *Rhizobium* into nodules, as happened in the case of *C. incana*, in which both populations were significantly lower in a shorter time exposure (Figure 5). This is because the oil alters the physical and chemical characteristics of the soil, causing the blockage of gas exchange with the atmosphere and affecting microbial populations. Furthermore, the weathered oil is adsorbed in the ground, being less accessible and more difficult to degrade by microorganisms [66, 67]. This may bring a direct impact on rhizobia, because they are aerobic bacteria that remain in

Some authors mention that soil conditions have a marked effect on rhizobia, because they can impact the survival and the infectivity of root hairs [68, 69]. However, there are many other factors that influence on effectiveness symbiosis such as specificity and virulence of the bacterium *Rhizobium*, nutrimental factors, soil temperature and pH [70], the latter is of utmost importance because has been reported to decrease significantly in contaminated soil [38]. Added to this there are other factors such as the accumulation of heavy metals and salts that

0.0E+00

**Figure 5.** Quantification of populations of *Rhizobium* extracts in nodules and Free Living Nitrogen Fixing Bacteria in

*L. leucocephala* almost doubled FLNFB populations with 79,457 mg.kg-1 of weathered oil. This can be explained because in tolerant species, some microorganisms can increase their popu‐ lations in the presence of hydrocarbons [71], allowing support microbial growth. As well as [72] argue that tolerant plants are promising tools to accelerate the removal of PAH in long term polluted soils, due to their ability to thrive in a contaminated site, and its success is probably influenced by the relative amount of exudates and other compounds within the root, that stimulate microbial growth [73]. On the other hand, several authors argue that changing

150 79,457 **Concentration (mg.kg-1)**

*Leucaena Leucocephala*

Rhizobium

Free living nitrogen fixing bacteria

5.0E+06

1.0E+07

1.5E+07

2.0E+07

2.5E+07

Soil is one of the most valuable resources that humanity has due to the variety of services offered and which depend for food. Currently, there is tremendous competition for land use, either for use as human settlements, commercial, industrial or produce raw materials (wood) and food, so weathered oil pollution is one of the most important challenges for scientists, because not only it is decontaminated, it comes to restoring the quality and safety of the soil, its properties and microbial component vital for production and proper plant development.
