## 1 Introduction  
To overcome the energy demands of the fast-growing industry and human population, the daily use of petroleum hydrocarbons (PHCs) is increasing alarmingly [1]. PHCs are a mixture of different compounds comprising aliphatic, aromatic, resins, and asphaltenes that are highly toxic to living beings [2,3]. The contamination of PHCs in the soil mostly occurs due to accidental spills, underground storage malfunction, extraction and transportation losses and causes a serious environmental threat to all living communities due to recalcitrance and the xenobiotic nature of PHCs [4,5]. The soil contamination with PHCs results in infertile soil because of hydrophobic conditions that ultimately lead to low plant and microbial growth [6]. PHCs affect physico-chemical properties of soils such as pH, electrical conductivity (EC), total exchangeable cation, soil moisture and oxygen contents that disrupt micro-climate around the soil and impairs plant growth by reducing soil porosity [7,8].  
Among different PHC remediation techniques, bioremediation--particularly microbe-assisted phytoremediation--is considered the most efficient and environmentally friendly remediation approach [9]. PHCs removal through microbe-assisted phytoremediation in soil is currently a promising strategy to reduce oil pollution [10]. The removal efficiency of PHCs can be enhanced by strengthening the plant rhizosphere [11]. Plant growth and development is directly improved by the microbial actions as siderophores production, phosphorus solubilization, nitrogen-fixation and ACC-deaminase activities [12,13]. In return, plant roots excrete enormous amounts of vitamins, organic acids, hormones, amino acids, mucilage, sugars and other substances that stimulate microbial activities in the rhizosphere [14,15]. Most of these root exudates have structural similarities with PHC aromatic fractions [16,17]. Thus, microbes use PHCs as a carbon source to increase their biomass and subsequently remove PHCs from soil [18].  
Although biochar addition to agricultural soils may pose potential risks to food safety due to the release of polycyclic aromatic hydrocarbons from the biochar [19], its addition to contaminated soils improves the properties of soil such as fertility, nutrient status, porosity, pH, water holding capacity and oxygen supply [20]. Biochar also minimizes the extent of pollutant's toxicity by precipitation, surface adsorption, sequestration and partitioning [21]. Furthermore, the addition of biochar also promotes microbial growth and their enzymatic activities by providing substrates and shelter for microbiota in the soil [22,23]. Hence, biochar and microbe addition could be helpful in minimizing pollutant toxicity, boosting plant growth and improving the removal of PHCs from the contaminated soil [23].  
Legumes can easily grow in different habitats including nutrient-deficient soils due to their nitrogen-fixing abilities [24]. Nitrogen fixation makes legume plants suitable for the restoration of soil health and fertility status of poor or contaminated lands [25]. Mungbean is an important legume crop and is ranked second among pulses in Pakistan [26]. The total area under mungbean cultivation in Pakistan is about 0.25 million hectares, with 178 tons of total production (grains) and with an estimated yield of 515 kg ha\({}^{-1}\)[27]. Although mungbean plants have the ability to grow in contaminated soils [28], studies reporting the effects of biochar and microbe addition on mungbean growth and PHC removal from diesel contaminated soil are still limited [22]. Therefore, the aim of this study was to examine the effects of biochar and _Enterobacter_ sp. MN17 addition on the removal of PHCs and growth of mungbean in diesel-polluted soil.