4.1.4 Fertilizer Effects on Depletion of Specific Petroleum Fractions
Fertilizer had no effect with P<0.20 (Table 2) except for the aromatic C>10-12, which showed a
significant effect (P=0.063) (Figure 14). The variability in the fertilized treatments was large, yet
fertilization resulted in lower degradation (P=0.063) of the aromatic C>10-12 fraction than the
non-fertilized treatments.
Inhibition due to fertilizer is counter-intuitive, yet it agrees with the general observations from
two demonstrations we conducted in Korea. These data suggest that fertilizer alone can inhibit
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.
4.1.5 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 15). Bacteria but not fungi responded to fertilization. Fungi but not bacteria responded to
plants (Figure 16). 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 17). These 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 16). 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.
21
Previous Page
