**7. Perspectives and conclusions**

biodegradation extent was evident in ageing but not in freshly spiked soil, which was consid‐ ered to be the result of the adaptation of indigenous bacteria *P. aeruginosa* by entering a stationary phase during the time of ageing (200 days) and by the subsequent production of surfactants. On the other hand, it was suggested that ageing of the soil is not the main parameter influencing PAH‐availability level, but the complexity of the organic constituents (i.e. coal tar, pitch, soot or coke) influence overall PAH availability in soil [33]. In addition, some bioremediation studies have evidenced the importance of the physicochemical param‐ eters of organic contaminants on the availability to microorganisms, which have effect on the biodegradation rate [27]. Soil properties and the indigenous microbial population affect the level of biodegradation; therefore, a detailed study on soil properties such as physicochemical

336 Soil Contamination - Current Consequences and Further Solutions

and biological parameters must be performed to select the bioremediation technique.

**effect on the response of microbial population to contaminants?**

**6. How does the impact of the agricultural management system have an**

The different responses of indigenous microorganisms to the PAHs degradation in agricultural contaminated soils are attributed mainly to the deficiency in nitrogen and phosphorous availability. As discussed above, organic matter plays a key role in the bioavailabilty of organic contaminants; however, the organic matter in the soil is also the primary source of essential nutrients such as nitrogen, phosphorous and sulphur [34], and it is often a carbon source easier to assimilate than the contaminant. Therefore, a good understanding of soil management systems can help to infer how soil microorganisms behave when facing to a contaminant. By studying the effects of soil management systems (no till and conventional tillage with se‐ quenced or rotation cropping) on the soil microbial community, it was found that an untilled soil and appropriate crop rotation systems favoured richness and diversity of the microbial community. Changes in microbial communities have also been observed in soils with different agricultural management systems, having a considerable impact on the biological activity of the soil [35]. Furthermore, it has been observed that variations in the microbial communities associated with soils are influenced by the type of land use and by time [36]. The leguminous crops contribute to enhance the organic matter levels resulting in small changes in bacterial populations [37]. Besides, the reducing tillage with retention of crop residues improves and preserves the diversity of bacterial communities [35]. On the other hand, soil enzymes are involved in the cycling of nutrients and they can react rapidly to make changes in soil derived from contamination or by the use of different management systems [38]. The activity of six soil enzymes (β‐1‐4‐glucosidase, L‐leucine‐aminopeptidase, β‐1‐4‐N‐acetylglucosaminidase, phenol oxidase, phosphatase and peroxidase) was correlated with the chemistry of soil organic matter in sites with different broad land use (agriculture soil, pine forest, hardwood forest and pasture). They found that biological process and soil texture correlate well with the chemistry of soil organic matter, suggesting that interactions between microbial communities and soil organic matter influence the soil carbon dynamics [39]. However, soil enzymes have been used as disturbance and quality indicators of contaminated ecosystems [40]. Besides, the soil nutrient status, microbial biomass nitrogen and enzyme activities in five different land‐use

The sorption phenomena, sequestering mechanisms, content and quality of organic matter and nutrient availability have a direct role in biodegradation success, together with the microbial metabolism and the biological interactions between the populations, which also play a major role. Many authors have reported some bioattenuation failures in contrast to biostimulation or bioaugmentation, thereby a proper creation of the environmental conditions may be sufficient to remove PAHs as discussed earlier. Therefore, the variation in biodegradation results obtained by several authors can be attributed to complex‐multiplex interactions between biological inter‐ or intra‐relationships, soil constituents, the physicochemical prop‐ erties of contaminants and the environmental conditions. They may stall or diminish the biological activity given by allochthonous or indigenous microbial. A proper understanding of the selection of indigenous or allochthonous microbial consortia, agricultural management systems, the quantity and quality of nutrients and the diversity of microbial communities in the contaminated soils must be envisaged and studied in detail, in order to increase our understanding about the complex physicochemical‐biological interactions between the microbial community and its environment. The addition of organic residuals combined with a specialized microbial consortium has the potential to enhance the degradation of such contaminants and may become a promising technology in the near future. However, a combined election of different bioremediation technologies may raise the costs and may become too expensive to use.
