Field experiment results
Two-way ANOVA for the growth rate data (Table 4) showed no significant
differences due to location or exposure to TPH, although there was a
tendency for plants in oil-polluted soils to elongate slightly more
quickly than in clean soils, and to grow faster in the South than in the
North (Burgan: 0.3 – 0.4 cm day-1; Bahra 0.15 – 0.2
cm day-1). Analysis of the final dry weight for shoots
alone, roots alone and whole plant biomass datasets (Table 4) showed a
significant treatment effect in every case, with plants grown in
oil-polluted plots showing significantly reduced biomass when compared
with plants in clean soils. However, location was not a significant
factor influencing either growth rate or final plant biomass.
As expected, there was quite considerable variation in the degree of TPH
pollution across the polluted experimental plots (in the range 40 – 350
mV.sec), but the difference between the two locations was not
significant (Table 4), and overall, the data are indicative of moderate
TPH pollution in the oil-contaminated treatment plots established in
both areas.
Discussion
The absolute values of PAH concentrations and TPH found in Kuwaiti
desert soils polluted by a one-off (though catastrophic) hydrocarbon
pollution event, which occurred 20 years prior to sampling, are
relatively low compared to values seen in some chronically long-term
hydrocarbon-polluted industrial sites (e.g. Smith et al. 2006).
Nevertheless, the results observed from both our field survey data, and
the experimentally assessed response of H salicornicum plants toin situ weathered oil-polluted soils suggest that the levels of
PAH pollution found in Kuwaiti desert soils in 2011 still have an
adverse effect on plant survival and growth. This was reflected in the
distribution of individual species and plant assemblages across the
studied areas.
The results of the classification exercise (Table 3) showed that desert
vegetation in Kuwait responds strongly to the presence of PAHs in the
soil with a general shift in species presence towards oil-pollution
indicator species at historically oil-damaged sites. This provides
evidence that the native vegetation present in oil-damaged desert
ecosystems can act as a good indicator of the degree of contamination
and stage of recovery of the ecosystem from oil-pollution.
There was substantial uptake and accumulation of some PAH compounds by
plants growing in polluted soils. Phenanthrene concentrations in the
leaf tissues of plants sampled in two polluted areas were up to 25%
higher on average than the mean values in soils for this PAH. The
difference was even more marked for fluorene, which was present in mean
concentrations exceeding 60 µg kg-1 plant dry weight
in leaf tissues sampled from one oilfield area, but was below the limit
of detection in soils sampled in all survey areas. Whether this
observation reflects substantial long-term (over a 20s period) removal
of fluorene (and to a lesser extent, phenanthrene) by plants from these
polluted soils, or is due to some other factors, requires further
investigation, but the results are broadly in agreement with similar
observations made in other PAH-polluted ecosystems (Meng et al. 2011;
Wagrowski & Hites 1997).
The use of suitable phytoremediation management procedures, such as
utilising native plant species and their associated root microflora, has
been shown elsewhere to provide a useful low-cost, ecologically-friendly
option to assist with pollution clean-up operations
(e.g.
Ramos & Maranhao 2009). According to Abdallah et al. (2020), some
native plants can grow directly over contaminated soils. Evidence from
heavy metal pollution studies, also, suggests that the use of
multispecies mixtures (such as naturally occur in native plant
communities) can improve the removal of contaminants from soil, compared
to using monocultures (Burd et al. 2000; Belimov et al. 2001; Sheng &
Xia 2006; Wu et al. 2006; Wenzel 2009). However, relatively little is
known about the details of phytoremediation processes and uptake
pathways involved in plant removal of PAHs from soils by desert plants
(e.g., Slaski & Archambault 2000; Meng et al.
2011),
A major practical advantage of using native plant species as
phytoremediators is the ease of obtaining seed and growing the plants
prior to transplanting into desired clean-up locations. However, given
that some native species such as Haloxylon are preferred species
for camel grazing
(Halwagy
& Halwagy 1974: Halwagy et al.
1982), protection
from grazing will be necessary for the success of a planting programme
for phytoremediation purposes. In addition, a suitable supply of
irrigation water during seedling establishment is probably a necessity,
unless sufficient rainfall can be predicted during this period.
The experimental and field-survey evidence from this study supports the
hypothesis that several native species in Kuwait, includingHaloxylon salicornicum, Cyperus conglomeratus andRhanterium eppaposum , can survive and grow successfully in
multispecies communities on desert soils polluted by weathered oil and
the associated mixture of PAHs. The presence of different plant
communities, though partially influenced by geographical factors, can
provide a good indication of the degree of damage and the stage of
recovery of the system from pollution. The three primary target species
examined here exhibit high capability to take up, and (to a varying
extent) bioconcentrate some, but not all, of the PAH compounds present
in these contaminated soils; once again providing evidence of their
potential for phytoremediation purposes Halophytes in particular
(Haloxylon salicornicum is one of them) have been studied before
as a potential bioaccumulator (Shaygan et al, 2018)
Further research may include examining the physiology of the three
primary target plant species (and other species with potential for
oil-pollution phytoremediation usage, in North Africa and the Middle
East) in order to ascertain in more detail the uptake mechanisms,
pathways and fates of petroleum hydrocarbons and their breakdown
molecules in these plants (Edwards 1986; Volkering et al. 1992; Harms et
al. 2003). The study is also needed to examine the role of
root-associated rhizosphere microflora (e.g., Juhasz & Naidu 2000;
Binet et al. 2000; Wenzel 2009) in the target species as a means of
enhancing phytoremediation impact.
Finally, the resulted reported herein may have implications to other
arid and semi-arid desert areas of North Africa and the Middle East in
which oilfields are located, and which are vulnerable to oil-pollution
incidents. . Our data clearly demonstrate that native desert plant
communities are useful indicators of the degree of damage, and recovery
from such pollution. Furthermore, some native phytoremediator species
appear to have good potential to assist in the restoration of
oil-damaged desert ecosystems.