Organic amendments for improving nickel phytomining with Odontarrhena serpyllifolia

Phytomining is an environmentally friendly, cost-effective plant-based technique to recover valuable metals from mineralised or polluted soils. Nickel phytomining has garnered much attention due to the vast number of Ni hyperaccumulator species identified in ultramafic areas worldwide and their promising metal extraction yields. Over the last few decades, the need to optimise phytomining yields through agronomic and crop management practices has been emphasised. Hence, this study evaluated the effects of organic amendments on phytomining yields using the Ni hyperaccumulator species Odontarrhena serpyllifolia, which is endemic to the Bragança and Morais ultramafic massifs in Portugal. The primary objective was to determine if the organic amendments biochar, and a combination of biochar with industrial sludge, and biochar with pig slurry, were appropriate for improving Ni harvestable amounts (governed by plant biomass and Ni accumulation in shoots) with O. serpyllifolia in the Morais massif. Plant seeds and soils were collected from the Morais massif. Organic amendments were added to the collected soil, including 1.5 % and 3 % biochar (w/w), and the same biochar rates were tested in combination with 1 % dried industrial sludge or 1 % pelletized pig slurry. Plants were cultivated in a laboratory greenhouse under controlled conditions. Soil treatments containing both biochar and sludge yielded the most promising results, particularly when the highest biochar rate was applied. This can be attributed to the significant improvement in soil properties and nutrient levels. The application of 1.5 % and 3 % biochar alone had no significant effect. Pig slurry mixed with biochar had a pronounced negative impact on plants, which can be explained by its high salinity levels. This study confirms that certain types of organic waste can be valorised and incorporated into phytomining systems.