Three model plant and three crop plant species were grown for three generations in sand and compost. Pots were inoculated with 10 % soil initially, and with 10% of growth medium from the previous generation in generations 2 and 3, keeping replicates separate for all three generations. The microbiome community structure of the plant rhizosphere in each generation was characterised using ARISA DNA fingerprinting and 454 sequencing. Rhizosphere bacterial and fungal communities are different from those in bulk soil and there are also differences in the microbial community between different plant species. Plants both select and suppress specific bacteria and fungi in the rhizosphere microbiome, presumably via composition of their root exudates. Two out of three most abundant bacteria selected in the rhizosphere were isolated. These isolates proved to possess plant growth promotion properties. Plants are able to “farm” the soil in order to enrich it with plant growth promoting rhizobacteria (PGPR) species. However, in some plant species rhizospheres, invasions of opportunists and pathogens take place, mimicking events in plant monocultures. Other experiments using this multi-replicate system allowed for statistical analysis of the influence of Arabidopsis and Medicago mutants on the rhizosphere microbiome. Three groups of Arabidopsis mutants were tested: plants unable to produce aliphatic glucosinolates, plants impaired in the PAMP-triggered immune response and plants unable and over-expressed in methyl halides production and one group of Medicago mutants which are impaired in the mycorrhization ability. All these plant genotypes, except those for methyl-halide production and one genotype involved in PAMP response, significantly altered the rhizosphere microbiome.

This thesis presents biochar state of the art and investigations into the environmental benefits and potential impacts of biochar application to soil. Specifically, the opportunity biochar has to increase concentrations of potentially toxic elements (PTE) and polycyclic aromatic hydrocarbons (PAHs) in soil was investigated and contextualised. Results indicated limited environmental impacts in this regard. The capacity of biochar to interact with organic compounds was studied in two contexts: PAHs absorption and partitioning; and with respect to bioavailability and potential deactivation of herbicides. Regarding PAH partitioning, sewage sludge biochar (SSBC) was established to be more efficient than sewage sludge (SS) in reducing the bioaccumulation of PAHs in Lactuca satuva L. grown in contaminated soil; while increasing significantly (p < 0.05) biomass yield, relatively to a soil only control. Regarding herbicides, biochar amended soil was observed to reduce herbicide extractability (< 2%). This extractability being far lower than that observed in the biochar free control soils (40% and 90%). 14C-radiorespirometry indicated that herbicide sequestration within biochar greatly reduced its bioavailability. Biochar influence upon weed survival indicated high biochar application rates (5%) to reduce the effectiveness of herbicides, suggesting that biochar incorporation in to soil at these levels could potentially undermine agriculture that relies upon herbicides. Finally, biochar was tested as microbial carrier. Rhizobacteria survival was established to be higher in biochar produced from redwood than in peat (a common microbial carrier) at high incubation temperatures (25°C and 35°C). In conclusion, biochar addition to soil presents limited direct environmental pollution impact. While biochar absorptivity may be beneficial in mitigating the bioavailability of organic contaminants this trait needs to be considered carefully in agricultural soils where herbicides are relied upon. Given the encouraging results regarding the potential for biochar to act as a microbial inoculant carrier, further research is warranted.

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