Influence of nitrogen forms and application rates on the phytoextraction of copper by castor bean (Ricinus communis L.)

Fertilization is an important agricultural strategy for enhancing the efficiency of phytoremediation in copper (Cu)-contaminated soils. In this study, the effects of nitrogen (N) forms, including ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N), on the growth, translocation, and accumulation of Cu in the tissues of Ricinus communis L. were investigated in pot and hydroponic experiments. The results demonstrated that higher biomass and N contents in plants were obtained when N was supplied as NO₃⁻-N rather than NH₄⁺-N. Application of N increased the Cu content in the roots of R. communis, with a higher content after NH₄⁺-N (53.10–64.20 mg kg⁻¹) than NO₃⁻-N (37.62–53.75 mg kg⁻¹) treatment. On the contrary, the levels of Cu translocation factors were much higher in NO₃⁻-fed plants (0.34–0.45) than in NH₄⁺-fed plants (0.28–0.38). The suggested amount of N for fertilizer application is 225 kg hm⁻², which resulted in the highest Cu content in R. communis and optimal plant growth. As the main Cu-binding site, root cell walls accumulated less Cu in plants treated with NH₄⁺-N compared with NO₃⁻-N. Additionally, NH₄⁺-N induced a higher malondialdehyde content and more severe root damage compared with NO₃⁻-N. In the leaf, a larger number of black granules, which could be protein and starch grains involved in the detoxification of Cu in R. communis, were present after NH₄⁺-N than NO₃⁻-N treatment. These results illustrate that N forms are especially important for Cu translocation and accumulation and that immobilization and transformation of Cu in roots were improved more by NH₄⁺-N than NO₃⁻-N. In conclusion, N fertilizers containing the appropriate forms applied at suitable rates may enhance the biomass and Cu accumulation of R. communis and thereby the remediation efficiency of Cu-contaminated soils.