**6. How does the impact of the agricultural management system have an effect on the response of microbial population to contaminants?**

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 patterns (nature forest, park, farmland, street garden and roadside tree) were compared, and it was found that soil quality and fertility were affected by urban land‐use patterns. Nutrients were scarce in urban soil and restricted the soil microbial biomass and enzyme activities (urease, protease and nitrate reductase) [38]. Soil enzymes are usually present in moderate or high levels in agricultural soils and they can be correlated to the bacterial diversity found in contaminated vs agricultural soils [41]. Dehydrogenase activity is a more sensitive parameter than urease activity to evaluate the combined toxic effect of metals and PAHs in soils, and these activities are dependent on the enzymatic concentrations [42]. However, enzymatic activity of dehydrogenase and fluorescein diacetate hydrolase has been found, by some authors, in PAH‐ contaminated soils, and it has been attributed to the gradual adaptation of microorganisms to contaminants and their utilization as a sole carbon and energy sources [43].

Otherwise, soils can be exposed to physical, chemical or biological degradation having an effect on the diversity of microbial communities. From the foregoing, a good agricultural management system may positively change the microbial diversity and improve the nutrient quality in soil as well as the metabolic variety of the microorganisms, leading to a favourable response in the removal of some contaminants. The response of microbial communities in an agricultural land used to grow wheat and sunflower was studied after the addition of diesel fuel. Despite the majority volatilization of aliphatic hydrocarbons, the soil microbial popula‐ tion was able to entirely remove the aliphatic hydrocarbons, and only 1% of the initial contaminant load in the soil remained after 400 days of monitoring. In addition, soil quality indicators (dehydrogenase activity and soil microbial biomass) decreased their values in the first 18 days; however, they recovered their original levels and then exceeded them, reaching a maximum value at the end of the study [44]. Agricultural management system impacts on the response of microbial population to contaminants by producing changes in the biological activity of the soil accelerating or delaying the biodegradation process, which should be considered as the relevant factor in the remediation at field level.
