Supplemented Biochar Mitigates the Ammonium Toxicity in Basil (Ocimum basilicum L.) Plants

Ammonium ( $${\text{NH}}_{4}^{ + }$$ ) toxicity adversely affects the basil growth ability and productivity. Biochar amendment is an alternative that used to minimize or reverse the disturbances caused by abiotic stresses. Nevertheless, little is known regarding the mitigating effect of biochar on the $${\text{NH}}_{4}^{ + }$$ -stressed basil. Thus, the present study was conducted to investigate whether biochar amendment could alleviate the $${\text{NH}}_{4}^{ + }$$ toxicity and determine how biochar helps reduce this degree. To this end, the basil plants were cultivated in a controlled environment (24°C/18°C, 14 h/10 h) and subjected to one of three $${\text{NH}}_{4}^{ + }$$ : $${\text{NO}}_{3}^{ - }$$ ratios (0 : 100, 50 : 50, and 100 : 0) at a constant N of 13 meq/L, corresponding with biochar treatments (3%, w/w) or without biochar. Chlorosis, stunted growth, and even foliage necrosis, were observed when the basil plants treated with 100% $${\text{NH}}_{4}^{ + }$$ nutrition. Concomitantly, interrupted photosynthesis and oxidative damage were also imposed by high $${\text{NH}}_{4}^{ + }$$ supply. By contrast, the presence of biochar significantly attenuated the $${\text{NH}}_{4}^{ + }$$ toxicity and, accordingly the growth was considerably improved for all three treatments. Additionally, biochar-amended basil plants showed significant increased photosynthesis-related parameters (net photosynthesis, the stomatal conductance, transpiration rates, and water use efficiency) and the antioxidative machinery (improved antioxidant enzymes and declined ROS accumulation and lipid peroxidation). Overall, our data enlightened the fertilization importance of biochar amendment, particularly pertaining to the basil plants at risk of $${\text{NH}}_{4}^{ + }$$ stresses.