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.