The rhizosphere effect refers to the positive influence of plant roots
on microbial population and activity in the rhizosphere (Mukerji et al.,
2006). In this study, alfalfa contribution to enhance microbial number
and lipase activity appeared to be limited, as the presence of vegetation
(phytoremediation and bioaugmentation-assisted phytoremediation
treatments) did not result in a statistically significant improvement,
with respectto unvegetatedsoil (naturalattenuation and bioaugmentationtreatments). As a result, theobserved increase in theremoval of petroleum hydrocarbons by alfalfa could be attributed to the action of
plant enzymes different to lipases or to an abiotic contribution of root
exudates enhancing desorption of hydrocarbons.
Contrary to expectations, this study did not find evident consistency
between the number of alkane-degrading microorganisms, soil lipase activity and TPH removal. At 90 days the following divergent trends were
obtained in 1) number of alkane-degrading microorganisms:
NA b PR b BA + PR b BA, 2) soil lipase activity: PR b NA b BA + PR b BA
and 3) TPH removal: NA b PR b BA b BA + PR. The three parameters
were higher in bioaugmented treatments that in non-bioaugmented
treatments. However, the presence of plants leads to a more variable
behaviour. The lack of a clear correlation indicates that the population of
microorganisms able to degrade alkanes and the soil lipase activity were
not the only factors determining the removal of TPH.