Introduction
The use of petroleum hydrocarbons for energy and raw materials in various applications is increasing significantly, leading extensive release of hydrocarbon contaminants to affect air, soil, surface water and groundwater. Phytoremediation has been an appealing technology to clean contaminated soil and water as it does not require extensive capital investment and can enhance soil properties. However, studies rarely explored the efficacy of phytoremediation in one of the world’s most polluted desert soils, caused by the deliberate destruction of oilfield installations during the First Gulf War in 1990-1991 (Hirschmann 2005; Omar et al. 2009).
In addition to possible previous land degradation and climate change impacts (Berdugo, et al 2020), the soil was contaminated by oil spillage from 798 sabotaged oil-wells. Moreover, formation water (over 10% salt content) and over 6 billion gallons of seawater (4% salt content) used to extinguish the burning oil wells flowed from the damaged wells and accumulated in the natural depressions of the landscape. The areas were termed ‘oil lakes’ (CIC, 2003). The damage resulted in contaminating over 40 million m3 of soil with oil and salt residues, resulting various types of oil contaminations including wet ‘oil lakes’ dry ‘oil lakes’, and oil-contaminated soil piles. Although the crude oil was mostly pumped out (Al-Daher et al., 1998), nonetheless the petroleum penetrated down 30 cm or deeper across large swathes of soil. The Kuwait Environmental Public Authority (KEPA: Beatona, Kuwait Official Environmental Portal, 2013) reported that oil lakes changed the soil texture, killed wildlife, and in some cases seeped into the soil layers reaching the only freshwater aquifer in northern Kuwait. This severely contaminated desert ecosystems continue to this day, and in the worst affected areas, it is thought that the pollution would likely persist for decades (Hirschmann 2005).
Over the years, the crude oil further broke down to smaller molecules such as polycyclic aromatic hydrocarbons (PAHs). Most PAHs are hydrophobic, display high stability, and can persist for decades in the environment (Wagrowski & Hites 1997; Hirschmann 2005). The amplified possibility of biomagnification due to their lipophilic nature increases the chance of persistence and trophic-level transfer in impacted ecosystems.
Plant species have different responses to, and tolerance of, oil contamination. Hence, that plant communities are usually changed by the impacts of such contamination with petroleum hydrocarbon-tolerant species becoming dominant once the initial high-toxic impacts subside to allow re-colonisation. An earlier study (Al-Ateeqi 2010) identified two native plant species growing in oil-polluted soils in Kuwait: Haloxylon salicornicum (Amaranthaceae) andCyperus conglomeratus (Cyperaceae). These species, along with a third plant considered as a possible phytoremediator species,Rhanterium epapposum (Asteraceae), are widespread in Kuwait (Halwagy & Halwagy 1974; Al-Shehabi & Murphy 2017). The three species are also present in other oil-producing regions of several North African and Middle Eastern countries (www.gbif.org/species/3758958; www.gbif.org/species/2714301; www.gbif.org/species/3110118).
Suitable phytoremediation management procedures require a good understanding of native plant species response to hydrocarbon contamination. Abdullah et al. (2020) assessed the ecosystem resiliency to total petroleum hydrocarbon (TPH) contamination in Kuwait using remote sensing and GIS. This study found that autogenic recovery of native desert plants occurred within a few years as 34% of the TPH contaminated areas were re-colonised with native desert plants. The contamination levels of TPH also changed over time and the variations could be significantly correlated with the soil type, vegetation type, geological substrates, geomorphological features, and annual precipitation. The study shed light on the succession process of vegetation survival and growth over TPH contaminated soils, but did not report the response of specific native desert communities to different TPH contamination at different concentration levels.
The primary goal of the present paper is to test whether indigenous species commonly found in oil-impacted habitats such as H. salicornicum species  could establish and grow successfully in desert soils contaminated with petroleum hydrocarbons. In our research, we assessed the hypothesis that some native plant communities in Kuwait are able to tolerate conditions in weathered oil-polluted soils containing oil by products (polycyclic aromatic hydrocarbons: PAHs), and that some of their component species may have potential as phytoremediator. The outcome of this study provides a useful low-cost, and ecologically friendly option by delineating a better understanding of utilizing native plant species to assist with clean-up operations in arid regions. These results are of especial importance to the United Nations general assembly declaration in 2019 of its 2021-2030 UN Decade of ecosystem restoration (Willemen et al., 2020).