ESTCP Project #1011, Rhizosphere
Final Report
the degradation on some petroleum fractions relative to control treatments (Reynolds et al.,
2001). Whyte et al., 1997, found Pseudomonas spp., isolated from cold soils could degrade C5 to
C12 aliphatics, toluene, and naphthalene at both 5 and 25 C, and also possessed both the alkane
and naphthalene degradation pathways. Their data indicated that both alkane and naphthalene
degradation capabilities, which are located on separate plasmids, can naturally coexist in the
same bacterium. Our earlier work at Fairbanks showed that the dominant culturable bacteria in
both control and fertilized soils were Pseudomonas spp. (Reynolds and Wolf, 1999). The
mechanisms for fertilizer inhibition of heavier fractions are not clear, but we have observed this
in several field studies.
4.3.6 Microbial Characterization. Because the potential for successful remediation of
petroleum-contaminated soils is determined by the number and activity of the hydrocarbon-
degrader microbial population in the soil, we also assessed the influence of fertilizer addition and
vegetation on culturable microbial numbers in a petroleum-contaminated soil at all three sites.
Using culturable microorganisms as a monitoring variable, significant treatment effects were
seen only at the Annette Island site. Soil samples were collected four times over a period of 20
months and total plate counts were used to enumerate bacteria and fungi. The bacterial numbers
significantly increased as a result of fertilizer addition and fungal numbers increased following
the establishment of vegetation (Figure 36). Bacteria but not fungi responded to fertilization.
Fungi but not bacteria responded to plants (Figure 37). The results indicated that adding fertilizer
and establishing vegetation increased microbial populations differentially and the potential for
biodegradation of the petroleum contaminants at the site. Motor oil, cyclohexanol and benzoic
acid degrader populations were determined using most probable number (MPN) methods. At 10
months, there was an increase in degraders for motor oil and cyclohexanol but a decrease for
benzoic acid degraders (Figure 38).
Phospholipid fatty acid data for Barrow show an increase in the fungal biomarker, n18:2w6c
(Figure 39). Plants increased fungal biomarkers at Barrow during the study. Non-planted
treatments did not show this effect. These data, combined with the Annette Island data, also
support the concept that one of the benefits of rhizosphere enhanced treatment is better
degradation of more recalcitrant compounds. Fungi have been shown to typically have greater
ability to degrade recalcitrant compounds (Donnelly and Fletcher, 1994) and the planted soils
have greater fungal numbers (Figure 37). This finding is also supportive of the chemical analyses
that showed a significant plant effect for depletion of the relatively recalcitrant compounds.
Additionally, the fertilizer effect on bacteria but not fungi suggests that one of the results of
fertilizer is an immediate or rapid bacterial response--which is fitting with bacterial growth rates
relative to fungi--and this may be at the cost of reduced degradation of petroleum. This may
explain in part the inhibition of depletion of some petroleum fractions associated with
fertilization that we have observed in our field studies.
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