Valuable non-food crops for biochar-assisted phytoremediation of contaminated soils: The case of cardoon, rapeseed and safflower

Biochar can affect the bioavailability of potentially toxic elements (PTEs) in contaminated soils thereby influencing plant growth. For this reason, biochar could be a resource for the assisted phytostabilisation of PTEspolluted soils. The aim of this study was therefore to assess the influence of a softwood-derived biochar (5 % w/w), added to soils (S1 and S2) contaminated with Cd (4.8 and 74 mg & sdot;kg- 1), Pb (318 and 1899 mg & sdot;kg- 1) and Zn (622 and 3803 mg & sdot;kg- 1), on selected soil biochemical and microbial features. The suitability of biochar in the assisted phytostabilisation in combination with cardoon, rapeseed and safflower was also evaluated. Overall, biochar decreased (or left unaffected) the number of culturable bacteria, actinomycetes and fungi in both soils. Soil enzyme activities were also reduced by biochar (e.g., -64 and 75 % for dehydrogenase and -16 % and 22 % for beta-glucosidase in S1- and S2 + Bio), while urease activity increased (+190 and 46 % in S1- and S2 + Bio). Actinobacteria and bacterial taxa involved in soil nitrogen cycling (e.g., Nitrospira) were enriched in biocharamended soils, while a marked loss of Cyanobacteria and taxa associated with oligotrophic conditions (e.g. Gaiellales, Sericithochromatia, Methylinbuim, Solimonadaceae, Gammaprotobacteriaceae and Salinisphaerales) was recorded. Increased root biomass of cardoon (+22 and 14 % in S1- and S2 + Bio compared to control plants), safflower (+71 and 98 % in S1- and S2 + Bio), and rapeseed (+17.1 + 2.4 % in S1- and S2 + Bio) was recorded in the amended soils. An increase of safflower and rapeseed shoots was also detected. In most cases, biochar addition reduced the PTEs concentration in plant roots/shoots/seeds favouring their build up (as total amounts) in the roots, as a consequence of enhanced growth due to biochar. Overall, the results indicated that safflower, rapeseed and cardoon can be used in assisted phytoremediation programmes of PTEs-contaminated soils, providing some valuable biomass that can generate income.