Research paperGrowth and metal uptake of canola and sunflower along a thickness gradient of organic-rich covers over metal mine tailings

•We grew biofuel crops on fertilized organic covers over metal mine tailings.•Thin organic covers (<15 cm) increased metal availability as a result of tillage.•Oilseed crops had limited uptake of nickel and copper over thin organic covers.•Oilseed biomass production was not affected on thin organic covers.•Thin organic covers over tailings produce biofuel crops with low environmental risks.

We completed a field-scale experiment to determine the thickness thresholds of covers of organic residuals required to grow biofuel crops over low-sulfur metal mine tailings with high metal content. An organic-rich mix made of municipal yard waste composts and wood waste was spread up to ∼70 cm thick over low sulfur Ni-Cu tailings. We seeded and fertilized biofuel crops of canola and sunflower across this gradient. We measured biomass production and concentrations of nutrients and trace metals in tailings, organic residuals, and plants. Below the threshold organic cover thickness of 15 cm, macronutrient content was reduced and bioavailable Fe, Ni and Cu were 5-50 times higher as compared to thicker organic covers, apparently as a result of tillage. Bioavailable K and Na increased by an order of magnitude and Mo doubled with increasing thickness of organic covers from 5 and 70 cm thick. The plants showed limited uptake of Ni and Cu, with bioconcentration factors of near 1 for sunflower and 0.6 for canola. Biomass production was not affected by the thickness of the organic cover. Plant rooting depth was deeper over thin organic covers, extending up to 15 cm into the tailings. Low stem Fe in plants over thin covers indicated a potential interaction between trace metals and Fe nutrition. These results support the use of covers of organic residuals as thin as 15 cm thick to grow biofuel crops over circumneutral metal mine tailings. Thin covers will make this approach more economical for mine reclamation managers.