Nowadays, production of biochar from agricultural wastes and its use for the amelioration of contaminated soils with heavy metals is increasing to reduce their negative effects on the growth of various plants. A factorial greenhouse experiment as a completely randomized design with three replications was performed to study the effect of different biochars on the spinach growth in a calcareous soil with different levels of nickel (Ni). The first factor consisted of biochars (control (CL), licorice root pulp (LRB), and rice husk (RHB) prepared at two pyrolysis temperatures (350 °C and 550 °C), each at 2.5% (w/w)), and the second factor included Ni application levels (0 (N0), 150 (N1), and 300 (N2) mg kg-1 soil). Shoot dry weight (15.0%), photosynthetic pigments (chlorophyll a (46.5%), b (39.5%), carotenoids (52.0%)), and micronutrients uptake (iron (51.8%), manganese (66.4%), copper (62.9%), and zinc (47.1%)) were decreased by increasing the Ni levels from N0 to N2. The activity of antioxidant enzymes (catalase (36.3%) and guaiacol peroxidase (29.3%)) and Ni uptake were increased by the Ni application levels. The biochar treatments (RHB350, RHB550, and LRB550) increased shoot dry weight of spinach through the reduction of Ni uptake, activity of antioxidant enzymes and enhancement of the micronutrients absorption. In general, the effect of the biochars produced at 550 °C on the reduction of Ni uptake and the increase in plant growth were better than the biochars produced at 350 °C. Finally, it could be concluded that the RHB550 is more suitable for reducing the Ni toxicity and improving the growth indices of the spinach.
Keywords: Antioxidant enzymes; Micronutrients uptake; Photosynthesis pigments; Pyrolysis temperature.