## 4 Discussion
### Soil Health  
In our study, the application of organic amendments, particularly dried cow slurry, enhanced the values of soil microbial parameters. Likewise, the biomass (DW) of _B. napus_ plants was approximately three-fold higher in pots amended with dried cow slurry, with respect to pots amended with composted horse manure and unamended controls. No clear differences were observed between soils amended with composted horse manure and dried cow slurry in terms of soil physicochemical properties (Table 3). On the other hand, the values of the C/N ratio were relatively similar for both amendments: 17 for composted horse manure (C: 34%, N: 2.0%) and 15 for dried cow slurry (C: 42%, N: 2.8%). Besides, as described above, the doses of these two organic amendments were adjusted so that the final OM content was equal in both cases, i.e., 20% OM. However, due to the inherent nature and state of stabilization (maturation) of the amendments used here, it is indeed likely that dried cow slurry contained a higher content of easily oxidizable OM, as well as of easily assimilated N, than composted horse manure, explaining its stronger stimulatory effect on plant growth and microbial biomass and activity. In agreement with this, all the parameters calculated here from Biolog EcoPlatesTM datawere affected by the type of organic amendment, with cultivable heterotrophic bacterial communities being more active, in terms of C substrate utilization rates, in soils amended with dried cow slurry (nonetheless, they showed a lower Shannon's diversity). It is not unusual to observe a reduction in soil microbial diversity (together with an increase in soil microbial biomass and activity) as a result of the application of organic amendments, probably due to the fact that the more efficient species in terms of their capacity to rapidly use easily metabolizable carbon substrates provided by the amendments outcompete those with a lower capacity, thus leading to a less diverse community. The values of Pielou's evenness, calculated from amplicon next generation sequencing data, showed a more uniform microbial community in soils amended with dried cow slurry (in turn, rarefied richness values were higher in soils amended with composted horse manure). Importantly, our clustering analysis (Figure 5) clearly separated unamended controls, soils amended with composted horse manure, and soils amended with dried cow slurry. According to our metabarcoding data, the three bacterial strains used here for bioaugmentation purposes in an attempt to stimulate the degradation of the target contaminants (_Burkholderia xenovorans_ LB400, _Paenibacillus_ sp., _Lysinibacillus_ sp.) did not thrive in the experimental soil, which could further explain the lack of contaminant degradation observed in our study. Indeed, the relative abundance of these three genera (_Burkholderia_, _Paenibacillus_, _Lysinibacillus_) did not increase in bioaugmented vs. non-bioaugmented soils. In fact, \"bioaugmentation\" was the least important factor explaining the variability of the data, as indicated by the variation partitioning analyses. Bioaugmentation is a bioremediation strategy which, in many cases, fails to deliver the intended reduction in contaminant concentrations due to the lack of survival and growth of the inoculated strains, owing to their poor competitive fitness under those specific environmental conditions. Thus, although bioaugmentation with _Rlodococcus erythropolis_ did initially enhance hydrocarbon degradation in soil, its effectiveness strongly decreased over time as the proportion of _Rlodococci_ was reduced from 25 to 1% of the total bacterial community (80 days after their inoculation), pointing out the difficulty to achieve the desired goal in bioaugmentation initiatives [62]. Other authors [63] also found that the bacteria inoculated to degrade PAHs in a contaminated soil decreased considerably in number or even disappeared over time (after approximately 100 days).