## 4 Bioremediation of PAH-Contaminated Soil by Composting
### Bioaugmentation  
When the indigenous microbial activity is not sufficient, or does not have the potential to achieve the set goals for bioremediation [112; 113], it appears imperative to accelerate the process using different approaches. Among these approaches is bioaugmentation. The mechanism of this approach fundamentally depends on introducing exogenous microorganism strains that are characterized by their high capacity and diverse metabolic profiles in degrading the target contaminants [16; 25; 29; 31; 32]. However, and as concluded in many studies, the application of this approach has not always been effective in enhancing biodegradable capacity, mainly during the composting of contaminated soils [25; 88; 114; 115]. For instance, 84% of petroleum hydrocarbon was degraded when _Candida catentulate CM1_ was used as an inoculant, while a removal rate of only 48% was obtained without inoculation, indicating a positive impact of bioaugmentation [29]. On the other hand, treatments using different substrates (mixing ratio = 1:1) were performed at the laboratory and field scales, and incubated with/without fungal inoculum (_Planerochaete velutina_). Laboratory scale treatment showed that HMW PAHs were degraded significantly in the fungal-inoculated microosoms, such that 96% of four-ring PAHs and 39% of five- and six-ring PAHs were removed in three months, whereas 55% of four-ring PAHs and only 7% of five- and six-ring PAHs were degraded in non-inoculated ones. However, the field scale achieved similar degradation rates. Importantly, the number of gram-positive, PAH-ring, hydroxylating dioxygenase genes in the field scale experiment was found to increase 1000-fold, indicating that bacterial PAH degradation played a major role [116]. [PERSON] et al. [117] compared bioaugmentation using _Acinetobacter SZ-1_ strain and biostimulation using (NH4)2 SO4 and KH2 PO4 in a petroleum-contaminated soil. It was found that the dissipation of total petroleum hydrocarbons (TPH) and the amounts of cultivable TPH, alkane, and PAH-degrading microorganisms were higher for biostimulation than for bioaugmentation. Similarly, [PERSON] et al. [118] demonstrated that fungal inoculation, including four well-known PAH-degrading microorganisms (_P. chrysosporium_ IMI 232175, _Coriolus versicolor_ IMI 210866, _Pleurotus ostreatus_ IMI 341687, and Wye isolate #7) in a mixture composed of non-sterile, coal-tar-contaminated soil and wheat straw, was unsuccessful in improving PAH removal. [PERSON] et al. [25] reported that the introduction of the white-rot fungi _T. versicolor_ ATCC 42530 was not able to improve the decomposition of PAHs; on the contrary, organic substrates were capable of achieving significant degradation rates. Furthermore, inoculation with _P. chrysosporium_ in a soil composting system was ineffective at enhancing the removal of benzo[a]pyrene [119].  
Actually, several biotic and abiotic barriers have been documented to be behind the failure of bioaugmentation, mainly during field application of this mechanisms [51; 120; 121; 122]. Biotic factors, including competition between indigenous and exogenous microorganisms for nutrients and the biodiversity of indigenous microorganisms, could act as a barrier to the invasion of exogenous microorganisms, in addition to antagonistic interactions and predation by protozoa and bacteriophages. Abiotic factors include all the physicochemical properties of pollutants and soils, such as pH, contaminant concentration, soil type, temperature, humidity aeration, nutrient content, and redox potential.