Interaction between biochar and Zn or K foliar fertilizer on the growth and Cd uptake of tobacco under cadmium stress

Objective Biochar and foliar fertilizers have been proved of effect on regulating heavy metal absorption and promoting growth of plants under heavy metal stress. This study evaluated the synergy of biochar and Zn/K foliar fertilizer, in order to find a better measurement for the efficient tobacco production in Cd- stress soil.

Methods Cd level of 3 mg/kg was added into a soil to stimulate slight Cd contamination in the pot experiment. The tested tobacco was Nicotiana tabacum L. cv. Zhongyan 100, a liable Cd bio-concentrated cultivar. There were 6 treatments: without wheat-straw made biochar or foliar fertilization (CK), base application of biochar 20 g/kg (B), foliar spray 0.5% ZnSO₄ (Zn), foliar spray 0.5% KH₂PO₄ (K), biochar + Zn spray (B + Zn) and biochar + K spray (B + K). The tobacco plant height, biomass were measured, and malonaldehyde (MDA) content and the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and Cd contents of plant parts were determined. Cd bio-concentration factor (BCF) and translocation factor (TF) were calculated. The contents of different Cd fractions in soils were analyzed.

Results 1) Foliar fertilization was more favorable to increase tobacco antioxidant abilities and facilitate tobacco growth than biochar did. Compared with B, the MDA contents of Zn and K treatments decreased by 12.8% and 15.1%, POD activities increased by 32.2% and 29.9%, CAT activities increased by 25.5% and 18.2% (P < 0.05). MDA content and antioxidant enzymes activities had no significant difference between Zn and K treatments (P > 0.05). Biochar application was more effective in decreasing bioavailability of soil Cd than Zn and K treatments. Compared with Zn and K treatments, the soil acid-soluble Cd of B treatment was decreased by 19.0% and 19.3%, reducible Cd by 22.3% and 23.9% (P < 0.05). Foliar fertilizers had no significant effects on soil Cd forms when compared to CK (P > 0.05). 2) Combination of biochar with foliar fertilizer had synergy effect. Compared with CK, B and Zn treatments, the plant height of B + Zn treatment increased by 52.8%, 25.9% and 11.8%, the shoot biomass by 146%, 89.1% and 45.3%, the SOD activity by 29.8%, 19.7% and 11.0%, POD activity by 94.0%, 64.1% and 24.1%, CAT activity increased by 183.8%, 90.9% and 52.2%; the Cd content in root decreased by 53.9%, 25.2% and 31.6%, in stem decreased by 74.5%, 53.9% and 63.7%, in upper leaves decreased by 63.1%, 18.8% and 40.1%, in middle leaves decreased by 63.6%, 27.7% and 35.3%, in lower leaves decreased by 63.8%, 34.1% and 46.9%; the Cd bio-concentration factor was decreased by 64.4%, 31.0% and 48.4%, the Cd translocation factor decreased by 33.3%, 20.2% and 28.5%; the soil acid-soluble Cd decreased by 19.5%, 7.5% and 19.0%, the soil reducible Cd decreased by 22.9%, 2.0% and 20.8%, the soil oxidizable Cd increased by 62.5%, 5.6% and 54.6%, the soil residue Cd was increased by 207%, 10.7% and 175%, respectively. Compared with B + K treatment, the shoot biomass of B+Zn treatment increased by 18.3%, the Cd contents of upper leaves, middle leaves and lower leaves decreased by 8.4%, 13.9% and 16.9%, the Cd bio-concentration factor and translocation factor reduced by 20.8% and 8.5%.

Conclusions In Cd stress soil, biochar could decrease the bioavailability of Cd, while Zn and K foliar application could increase the antioxidant abilities and promote the resistance to Cd and growth of tobacco plants, and the combination of them shows significant higher synergy in the beneficial effect than those did the individual use. Under the tested conditions, combination of biochar with ZnSO₄ performs better than with KH₂PO₄ did, showing promising measurement for efficient and safe tobacco production.