Microbial inoculants alleviate the adverse effects of Cu-contaminated soils amended with biochar on sunflower growth

Soil contamination is a pressing global issue driven by various anthropogenic activities, such as mining. This study evaluated the use of biochar and microbial inoculants as phytoremediation allies in promoting sunflower growth in a Cu-contaminated mining soil. Sunflower seedlings were planted in a Cu-contaminated mining soil amended with increasing doses of biochar (0 %, 2.5 %, and 5 % w/w) under greenhouse conditions. Seedlings were singly and co-inoculated with the bacterial strain Pseudomonas reactans EDP28 and the commercial arbuscular mycorrhizal fungi (AMF) Rhizophagus irregularis. The addition of 2.5 and 5 % of biochar to the Cu-contaminated mining soil significantly reduced sunflower shoot biomass by 49 % and 46 %, respectively, and root biomass by 63 and 50 %, respectively. This decrease is likely attributed to increased Cu accumulation in plant tissues, particularly in the roots (on average +38 %), driven by the enhanced availability of Cu in the soil. However, microbial inoculation, particularly the combined application of the bacterial strain and the AMF (Mix treatment), significantly supported sunflower growth and resilience under metal stress conditions. Mix treatment improved root elongation, root biomass, and shoot biomass by 48 %, 143 %, and 122 % at 2.5 % biochar, and by 45 %, 54 %, and 137 % at 5 % biochar, respectively. This was achieved by improving chlorophyll content and nutrient use efficiency. The beneficial effects were clearer in soils without biochar addition, where inoculation fully promoted sunflower growth. In contrast, in biochar-amended soils, inoculation helped to partially counteract the negative effects of biochar on plant development. This study demonstrates that sunflowers can effectively tolerate and accumulate high levels of Cu in their tissues, making them a promising candidate for phytoremediation strategies in mining areas, especially when aided by microbial inoculants, whilst the role of biochar in phytoremediation requires further investigation. Biochar can facilitate metal accumulation, but its impact on plant growth needs careful management. Future research should focus on optimizing the application rates and combinations of biochar and microbial inoculants to maximize phytoremediation efficiency and minimize any adverse effects on plant growth.